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The is a mounting evidence that global emissions of dust were significantly higher during glacial than interglacial periods of the Pleistocene, and probably the Pliocene epochs. this pattern is observed in records from the low and mid latitudes, albeit with a varying degree of amplitude. During these time periods spanning 4 million years, the Earth climate underwent major transitions, such as the initiation of the Northern Hemisphere Glaciations and the Mid Pleistocene Transition. In parallel, dust transport and deposition on the oceans might have underwent stepwise increases, mainly during glacials. However, it is not clear yet if such changes are representative of global or regional climate response. Thus, dust records in marine sediments reflect changes in the different processes that drive the emission, transport, and deposition of dust on the oceans. In here, we report a compilation of marine dust records spanning the Pliocene-Pleistocene from all the major ocean basins. The synthesis of dust records on a global scale allows the identification of common patterns of variability and drivers. We analyse the data to infer changes in the global atmospheric circulation on orbital time scale, and to assess its meridional and zonal response during major climate transitions since the Pliocene.

We have developed and applied new analytical methodologies to study the drivers of the spatial distribution of pyrogenic biomarkers throughout the Iberian Peninsula. The goal of the study is to develop a multiproxy pyrogenic biomarker approach to quantitatively reconstruct the paleo-occurrence and extension of biomass wildfires from sediments. For this purpose, we have compiled an extensive collection of lacustrine sediments throughout Spain. The sample suite is representative of a wide range of climates and ecosystems. We have quantified the abundance of different types of pyrogenic biomarkers such as BPCAs (benzene polycarboxylic acids derived from the chemical oxydation of pyrogenic carbon), MAS (monosaccharide anhydrides, namely levoglucosan) and PAHs (polyaromatic hydrocarbons namely with 4 or more rings). The data obtained, as concentrations or ratios, has been mapped and compared to the documented occurrence and extension of wildfires in Spain over the last 5 decades. Specifically, we have calculated the burned area within different radius around the lake, and correlated them against the biomarker data. The regional distribution of fires in Spain is the primary driver of the spatial distribution of pyrogenic biomarkers in lakes sediments. Thus, the sedimentary concentration of pyrogenic biomarkers, generally, correlates with the area of burned forest land within a radius of 10-20km. Work is still under way to decipher the meaning between the correlations of different types of pyrogenic biomarkers, and the relation between biomarker ratios and fire regimes and vegetation types. Our study shows that pyrogenic biomarkers in lakes can be used as paleoproxies to study regional fires, as opposed to those occurring at larger spatial scales. It remains to be seen, how our findings from Spanish lakes can be extrapolated to other continental regions. Nonetheless, our study validates their use to quantify the area of forest burned in the region surrounding the lake, in addition to the frequency of fires in the past.

The occurrence of ice-free conditions in the Nordic seas during the Last Glacial Maximum has been demonstrated using microfossil and biomarker approaches. An accurate picture of the prevailing oceanographic conditions may be more difficult to ascertain. Thus, uncertainties for most proxies increase at the cold extreme of temperature calibrations and glacial sediments contain small amounts of microfossils and biomarkers. Here we discuss sea surface temperature estimates derived from alkenone indices for the Last Glacial Maximum in the Nordic seas that provide a surprising scenario, where surface temperatures were warmer than at present. We conclude that the UK37′ and UK37 estimates may not be interpreted in terms of sea surface temperature as several factors may have conspired to bias the molecular signal. However, using an alternative index based on the abundance of the C37∶4 alkenone, we estimate a maximum temperature of 6°C.

To appraise the application of isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) as paleoclimate proxies in small lakes we investigated the effect of seasonal variability in temperature and dissolved oxygen concentration (DO) on the GDGT contents and its proxy in the water column of an oligotrophic karst lake in the Catalan pre-Pyrenees (Lake Montcortes). From October 2013 to November 2014, we collected suspended particulate matter (SPM) monthly at three depths and retrieved the sediment from a trap located at 20 m below the lake surface. The GDGT contents in the SPM and sediment trap presented a marked seasonal variation, showing the highest values in summer and autumn. In addition, the isoGDGT proxy TEX86 in SPM and sediment trap showed a marked seasonal variability which matched the temperatures of the upper water column and fit the published global trend between TEX86 and mean annual lake surface temperature (LST). However, in the hypolimnion where oxygen was depleted for most of the study period, we found that TEX86 co-varied with DO. This further confirms previous claims with seasonal field data that the sedimentary TEX86 LST signal may be confounded by DO conditions. Consequently, the application of TEX86 to estimate LST should be carefully appraised in environments with depleted DO conditions., This work was supported by the Spanish Ministry of Research through projects TETRACLIM (ref. CGL2010-15000), Montcortes-500 (ref. CGL-2012-33665); and ICTA-UAB's Maria de Maeztu award (MDM-2015-0552); and a pre-doctoral research studentship to MCT (FPI, BES2013-065846). MC was funded with a doctoral research studentship from the China Scholarship Council (CSC). Fieldwork was performed with the collaboration of E. Safont, N. Canellas and J. Goma. BES2013-065846Fieldwork permits were provided by the Territorial Service of the department of Agriculture, Livestock, Fishing and Natural Environment of Catalunya. Meteorological data in Pobla de Segur were provided by J. Sigro. We also thank N. Moraleda, C. Huguet and the UAB Analytical Service for technical support.

The southeast Atlantic Ocean is dominated by two major oceanic systems: the Benguela Upwelling System, one of the world's most productive coastal upwelling cells and the Agulhas Leakage, which is important for transferring warm salty water from the Indian Ocean to the Atlantic Ocean. Here, we present a multi-proxy record of marine sediments from ODP Site 1087. We reconstruct sea surface temperatures (U37 K′ and TEX86 indices), marine primary productivity (total chlorin and alkenone mass accumulation rates), and terrestrial inputs derived from southern Africa (Ti/Al and Ca/Ti via XRF scanning) to understand the evolution of the Southeast Atlantic Ocean since the late Pliocene. In the late Pliocene and early Pleistocene, ODP Site 1087 was situated within the Benguela Upwelling System, which was displaced southwards relative to present. We recognize a series of events in the proxy records at 3.3, 3.0, 2.2, 1.5, 0.9 and 0.6 Ma, which are interpreted to reflect a combination of changes in the location of major global wind and oceanic systems and local variations in the strength and/or position of the winds, which influence nutrient availability. Although there is a temporary SST cooling observed around the initiation of Northern Hemisphere glaciation (iNHG), proxy records from ODP Site 1087 show no clear climatic transition around 2.7 Ma but instead most of the changes occur before this time. This observation is significant because it has been previously suggested that there should be a change in the location and/or strength of upwelling associated with this climate transition. Rather, the main shifts at ODP Site 1087 occur at ca. 0.9 Ma and 0.6 Ma, associated with the early mid-Pleistocene transition (EMPT), with a clear loss of the previous upwelling-dominated regime. This observation raises the possibility that reorganisation of southeast Atlantic Ocean circulation towards modern conditions was tightly linked to the EMPT, but not to earlier climate transitions.

In the Quaternary paleosciences, the rationale behind analogical inference presupposes that former natural changes can be explained by causes operating now, although their intensity and rates can vary through time. In this paper we synthesise synthetize the results of different modern analog studies and discuss their value to obtain the best inferences from high resolution past records. This synthesis is based on the following: 1) The monthly monitoring of calcite precipitation reveals a strong connection with primary producers and between-years variability; this precipitation produces a seasonal signal with imprint on varve formation. 2) Clear pollen sedimentation peaks occur in spring/summer and fall/winter that coincide with temperature, precipitation, relative humidity and winds; this pattern converges with the two-layer coupled varves representing the same seasonality. 3) We assess the lake’s contemporary oxygenation dynamics over a three- year period; a combination of sedimentary REDOX proxies revealed different scenarios of oxic/anoxic shifts since 1500 CE. 4) We investigate presence of seasonality in the production/distribution of glycerol dialkyl glycerol tetraethers and derived temperature estimates in soils and particulate matter. Branched glycerol dialkyl glycerol tetraethers signatures and some derived temperature estimates proxies appear to mainly depend on the non-seasonal shifts in soil properties. 5) Currently we examine relationships and similarities between extant phytoplankton and derived pigments in water and traps, and their correspondence with subfossil pigments; some preliminary results are presented here.Keywords: high resolution, endogenic varves, calcite precipitation, pollen sedimentation, meromixis, freshwater glycerol dialkyl tetraether, subfossil pigments, long-term ecology.

Unidad de excelencia María de Maeztu MdM-2015-0552 The increasing influence of Atlantic Water (AW) in the Barents Sea, a process known as "Atlantification", is gradually decreasing sea ice cover in the region. Ongoing global climate warming is likely to be one of its drivers, but to further understand the role of natural variability and the biogeochemical impacts of the inflow of AW into the western Barents Sea, we reconstructed sea surface temperatures (SSTs) and primary productivity in Storfjordrenna, a climatically sensitive area south of Spitsbergen, between approximately 13,950 cal yr BP and 1300 cal yr BP. The alkenone U proxy has been applied to reconstruct SSTs, and the alkenone accumulation rate in marine sediments has been used to infer changes in primary productivity. Our data show that the SST increase was concomitant with the progressive loss of sea ice cover and an increase in primary productivity in the western Barents Sea. We interpret these changes as reflecting the increasing influence of AW in the area as the ice sheets retreated in Svalbard. The transition from the Arctic to the Atlantic domain first occurred after 11,500 cal yr BP, as the Arctic Front moved eastward of the study site but with considerable variability in surface ocean conditions. High SSTs at approximately 6400 cal yr BP may have led to limited winter surface cooling, likely inhibiting convective mixing and the return of nutrients to the euphotic zone and/or enhanced organic matter consumption by zooplankton due to an earlier light signal in the ice-free Storfjordrenna. During the late Holocene (3600-1300 cal yr BP), low insolation facilitated sea ice formation and thus brine production. The former may have launched convective water mixing and increased nutrient resupply to the sea surface, consequently enhancing primary productivity in Storfjordrenna. We propose that, on the basis of the paleoceanographic evidence, the modern increasing inflow of warm AW and the disappearance of pack ice on the Eurasian continental shelf are likely to weaken convective water mixing and decrease primary production in the region.

Collective assessment of marine and terrigenous organic biomarkers was performed on a sediment core spanning the last 34,800 years on the upper slope southeast of Brazil to verify the signatures of climatic variations in sea surface temperature (SST), marine productivity, and the flux of terrigenous material in this region. This evaluation is based on marine and terrigenous proxies including alkenones, chlorins, aliphatic hydrocarbons, n-alcohols, and fatty acids. This first report of organic biomarker data for this region confirms a correlation between SST, changes in terrigenous organic matter flow into the ocean, and marine productivity over the last 34.8 ka as a response to the displacement of the Brazil Current. Conditions prevailing during marine isotopic stage (MIS) 3 may be considered intermediate between the last glacial maximum (LGM) and the Late Holocene. For MIS 2, a period of low relative sea level, it was verified that the lowest SSTs were associated with the LGM and higher marine productivity. SST increased by up to 4.4 °C between the LGM and the Holocene. This reveals synchronicity between SST on the southeastern Brazilian upper slope and the North Atlantic Ocean SST records reported in earlier studies.

The Late Pliocene has been proposed as a possible analogue for understanding future climate change and for testing climate models. Previous work has shown that during the Pliocene the major upwelling systems were relatively warm, and that this meant they were either inactive, contracted, or were upwelling warmer waters than present. Here, we examine evidence from a site located on the margins of the modern Benguela Upwelling system to test whether the upwelling cells had migrated or contracted relative to present during the Pliocene. We applied several organic geochemistry proxies and foraminiferal analyses to reconstruct the Pliocene history of ODP Site 1087 (31°28′S, 15°19′E, 1374 m water depth), including the UK37′ and TEX86 indices (for reconstructing sea surface temperatures), phytoplankton biomarker concentrations and stable isotope ratios (for estimating export primary productivity, and for oxygen isotope statigraphy), and planktonic foraminifera assemblage abundances (for inferring water mass changes). These proxies show that, between 3.5 and 3.0 Ma, the southern Benguela region was cooler than the northern Benguela region by 5 °C, the latter being where the main upwelling cells are found today. From the multiproxy data obtained, we also infer that more extensive upwelling was present in the southern Benguela region during the Pliocene than at present, and that the Benguela Upwelling cells shifted northwards after the Pliocene epoch as a result of changes in the local wind field. We also find evidence that the Benguela Upwelling was sensitive to the pronounced cooling during the M2 and KM2 glacial stages, potentially associated with the expansion of sea ice and cooling in Antarctica in the Late Pliocene.

TEX86 (TetraEther indeX of tetraethers consisting of 86 carbon atoms) is a sea surface temperature (SST) proxy based on the distribution of archaeal isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs). In this study, we appraise the applicability of TEX86 and View the MathML sourceTEX86L in subpolar and polar regions using surface sediments. We present TEX86 and View the MathML sourceTEX86L data from 160 surface sediment samples collected in the Arctic, the Southern Ocean and the North Pacific. Most of the SST estimates derived from both TEX86 and View the MathML sourceTEX86L are anomalously high in the Arctic, especially in the vicinity of Siberian river mouths and the sea ice margin, plausibly due to additional archaeal contributions linked to terrigenous input. We found unusual GDGT distributions at five sites in the North Pacific. High GDGT-0/crenarchaeol and GDGT-2/crenarchaeol ratios at these sites suggest a substantial contribution of methanogenic and/or methanotrophic archaea to the sedimentary GDGT pool here. Apart from these anomalous findings, TEX86 and View the MathML sourceTEX86L values in the surface sediments from the Southern Ocean and the North Pacific do usually vary with overlaying SSTs. In these regions, the sedimentary TEX86-SST relationship is similar to the global calibration, and the derived temperature estimates agree well with overlaying annual mean SSTs at the sites. However, there is a systematic offset between the regional View the MathML sourceTEX86L-SST relationships and the global calibration. At these sites, temperature estimates based on the global View the MathML sourceTEX86L calibration are closer to summer SSTs than annual mean SSTs. This finding suggests that in these subpolar settings a regional View the MathML sourceTEX86L calibration may be a more suitable equation for temperature reconstruction than the global calibration.

A feature of Pliocene climate is the occurrence of “permanent El Niño-like” or “El Padre” conditions in the Pacific Ocean. From the analysis of sediment cores in the modern northern Benguela upwelling, we show that the mean oceanographic state off Southwest Africa during the warm Pliocene epoch was also analogous to that of a persistent Benguela “El Niño”. At present these events occur when massive southward flows of warm and nutrient-poor waters extend along the coasts of Angola and Namibia, with dramatic effects on regional marine ecosystems and rainfall. We propose that the persistent warmth across the Pliocene in the Benguela upwelling ended synchronously with the narrowing of the Indonesian seaway, and the early intensification of the Northern Hemisphere Glaciations around 3.0–3.5 Ma. The emergence of obliquity-related cycles in the Benguela sea surface temperatures (SST) after 3 Ma highlights the development of strengthened links to high latitude orbital forcing. The subsequent evolution of the Benguela upwelling system was characterized by the progressive intensification of the meridional SST gradients, and the emergence of the 100 ky cycle, until the modern mean conditions were set at the end of the Mid Pleistocene transition, around 0.6 Ma. These findings support the notion that the interplay of changes in the depth of the global thermocline, atmospheric circulation and tectonics preconditioned the climate system for the end of the warm Pliocene epoch and the subsequent intensification of the ice ages.

Two commonly used proxies based on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) are the TEX86 (TetraEther indeX of 86 carbon atoms) paleothermometer for sea surface temperature reconstructions and the BIT (Branched Isoprenoid Tetraether) index for reconstructing soil organic matter input to the ocean. An initial round-robin study of two sediment extracts, in which 15 laboratories participated, showed relatively consistent TEX86 values (reproducibility +/- 3-4 degrees C when translated to temperature) but a large spread in BIT measurements (reproducibility +/- 0.41 on a scale of 0-1). Here we report results of a second round-robin study with 35 laboratories in which three sediments, one sediment extract, and two mixtures of pure, isolated GDGTs were analyzed. The results for TEX86 and BIT index showed improvement compared to the previous round-robin study. The reproducibility, indicating interlaboratory variation, of TEX86 values ranged from 1.3 to 3.0 degrees C when translated to temperature. These results are similar to those of other temperature proxies used in paleoceanography. Comparison of the results obtained from one of the three sediments showed that TEX86 and BIT indices are not significantly affected by interlaboratory differences in sediment extraction techniques. BIT values of the sediments and extracts were at the extremes of the index with values close to 0 or 1, and showed good reproducibility (ranging from 0.013 to 0.042). However, the measured BIT values for the two GDGT mixtures, with known molar ratios of crenarchaeol and branched GDGTs, had intermediate BIT values and showed poor reproducibility and a large overestimation of the true (i.e., molar-based) BIT index. The latter is likely due to, among other factors, the higher mass spectrometric response of branched GDGTs compared to crenarchaeol, which also varies among mass spectrometers. Correction for this different mass spectrometric response showed a considerable improvement in the reproducibility of BIT index measurements among laboratories, as well as a substantially improved estimation of molar-based BIT values. This suggests that standard mixtures should be used in order to obtain consistent, and molar-based, BIT values.

[1] Marine biological export productivity declined in high-latitude regions in the North Pacific and Southern Ocean 2.7 million years ago, in parallel with the intensification of Northern Hemisphere glaciation. Here we present data from the North Atlantic, which show a similar but time-transgressive pattern of high-latitude productivity decline from 3.3 to 2.5 Ma, with productivity decreasing first at 69°N, hundreds of thousands of years before it declined at 58°N. We propose that the cumulative data are best explained by an equatorward migration of the westerly winds, which caused a southward shift in the zone of Ekman divergence and upwelling-associated major nutrient supply over this time interval. We suggest that a similar equatorward migration of the westerly winds may also help explain the productivity changes observed in other high-latitude regions, particularly the Southern Ocean. At 2.7 Ma, equatorial and temperate Atlantic sites began to show orbitally paced productivity pulses, consistent with a shoaling and meridional contraction of the nutrient-poor “warm sphere” that characterizes the low latitude upper ocean. This timing coincides with observed productivity changes in Southern Ocean, consistent with previous findings that the Southern Ocean exerts a strong influence on the fertility of the low-latitude Atlantic. Finally, we propose that the unique basin geometry of the North Atlantic caused deep water formation in this region to remain relatively stable despite equatorward migration of winds and ocean fronts.

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) of purportedly terrestrial origin are frequently detected in marine sediments, even in remote ocean sites where no direct impact from land erosion via rivers takes place. At these places, the most likely explanation for the presence of brGDGTs is in situ production or eolian transport, but neither possibility has been demonstrated for the open ocean. Here, we report the presence of isoprenoid (iso) and brGDGTs in eight dust samples collected off Northwest Africa. Based on previous studies, prevailing wind patterns, bulk chemistry, n-alkane composition and isotopic signatures, we show that Northwest Africa is the likely principal origin of the GDGTs in the dust. The concentrations of plant wax n-alkanes in the dust are several orders of magnitude higher than those of GDGTs and, based on the distributions of these two compound classes, we infer that they tag different carbon pools and sources of organic matter. Our finding demonstrates that brGDGTs and isoGDGTs in marine sediments and wind-derived deposits can have an eolian source. Consequently, climate reconstruction may be attempted from wind-derived deposits of brGDGTs, even in remote oceanic areas.

mid-Pleistocene climate transition (MPT) is defined by the emergence of high amplitude, quasi-100 ka glacial-interglacial cycles from a prior regime of more subtle 41 ka cycles. This change in periodicity and amplitude cannot be explained by a change in 'external' astronomical forcing. Here, we review and integrate published records of sea-surface temperatures (SSTs) to assess whether a common global expression of the MPT in the surface ocean can be recognized, and examine our findings in light of mechanisms proposed to explain climate system reorganization across the MPT. We show that glacial-interglacial variability in SSTs is superimposed upon a longer-term cooling trend in oceanographic systems spanning the low-to high-latitudes. Regional variability exists in the timing of the onset and magnitude of cooling but, in most cases, a long-term cooling trend begins or intensifies from similar to 1.2 Ma (Marine Isotope Stage, MIS, 35-34). The SST cooling accompanies a long-term trend towards higher global ice volume as recorded in benthic foraminifera delta O-18, but predates a step-like increase in delta(18)Oat similar to 0.9 Ma (MIS 24-22) that is argued to reflect expansion of continental ice-sheets. The strongest expression of Pleistocene cooling is found during glacial stages, whereas minor or negligible trends in interglacial temperatures are identified. However, pronounced cooling during both glacial and interglacial maxima is evident at 0.9 Ma. Alongside the long-term SST cooling trends, quasi-100 ka cycles begin to emerge in both the SST and delta O-18 records at 1.2 Ma, and become dominant with the expansion of the ice-sheets at 0.9 Ma. We show that the intensified glacial-stage cooling is accompanied by evolving pCO(2), abyssal ocean ventilation, atmospheric circulation and/or dust inputs to the Southern Ocean. These changes in diverse environmental parameters suggest that glacial climate boundary conditions evolved across the MPT. In turn, these modified boundary conditions may have altered climate sensitivity to orbital forcing by placing pre-existing ice-sheets closer to some threshold of climate-ice sheet response.

The seasonality of sea surface temperatures (SST) estimated from the alkenone- U K ′ 37 index has been a debated issue since the development of the proxy. Using a compilation of sediment trap time series data from 34 sampling locations, we show that the seasonality of maximum alkenone flux in sediment traps varies markedly across the oceans, depending not only on latitude and light availability but also on local oceanographic conditions. The seasonality of the alkenone flux to sediments may also be shaped by the complexity of sedimentation processes and a consistent, globally applicable, seasonal pattern is not apparent. Nevertheless, U K ′ 37 values display a world ocean scale correlation with mean annual SSTs (0 m) that closely resembles the standard calibration equation now established for modern surface sediment records. Thus, with a few notable exceptions at oceanographic locations proximate to major hydrographic fronts, it can be concluded that the integrated sedimentation patterns for U K ′ 37 measured in sediment trap time series provide a measure of annual mean SST.

The Arctic has undergone significant changes in the last 20–30 years. The understanding of these changes is limited by the time span or resolution of existing climate records. The main gateway of heat to the Arctic Ocean is the Fram Strait, between Greenland and Svalbard. We have reconstructed past air and sea surface temperatures and relative variations in freshwater extent in a marine core from eastern Fram Strait which spans the last 2000 years. The results show a progressive increase in air temperature and a decrease in the upper mixed water layer temperature, together with an increase in freshwater content during the last 1000 years. Considering the oceanographic features of the eastern Fram Strait, these results suggest that there might have been a progressive increase in the inflow of Atlantic water during the last 1000 years, accompanied by an increase in atmospheric heat transport, causing progressive sea-ice melting and subsequent cooling and freshening of the upper mixed water layer in this area. We suggest that our data show long-scale average changes in the modes of positive and negative Arctic multidecadal variability over the last 2000 years.

Changes in Southern Ocean hydrography may have played an important role in the Middle Pleistocene Transition (MPT), particularly through their impact on ocean circulation and atmospheric CO2 concentrations. Here we present foraminiferal Mg/Ca and δ18O results for the subsurface dwelling planktonic foraminifer Neogloboquadrina pachyderma (sinistral) at the Ocean Drilling Program (ODP) Site 1090. Results are used to reconstruct upper ocean temperatures and derive seawater δ18O in the Subantarctic Atlantic Ocean during the MPT. The new records indicate that, starting at ∼1250 ka, glacial temperatures and local (ice volume corrected) seawater δ18O in the upper water column of the Subantarctic Atlantic Ocean decreased, pointing to cooler (∼2 °C) and fresher (∼0.4‰) conditions than in the preceding glacial stages. These upper ocean hydrographic changes broadly coincide with the increase in the power of the 100 ky glacial–interglacial cycle in both records and with a shift towards decreased deep ventilation in the glacial Southern Ocean. Our finding suggests that an increase in Southern Ocean stratification, driven by the observed freshening of the upper water column, may have reduced the exchange of carbon between the deep Southern Ocean and the atmosphere during glacial stages. This process may have contributed, in combination with other mechanisms, to lower glacial atmospheric CO2 concentrations during the MPT.

The endorheic Lake Van in eastern Anatolia (Turkey) is the world’s largest soda lake and it is an important site in paleoclimate studies to understand past continental conditions in western Asia. In order to gain further insights into the biomarker signatures in Lake Van’s sediments we have analyzed particulate material in sediment traps deployed between August 2006 and July 2007. The biomarkers used were long chain alkenones (LCAs C37–C39, haptophyte lipids), isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs, Archaea membrane lipids) and pigments (chlorins and fucoxanthin). The biomarker fluxes indicate a strong seasonality in export primary productivity and the phytoplankton community structure. The highest total mass and organic carbon fluxes were found in summer, coupled to strong stratification while the lowest mass fluxes occurred in winter at the time of water column mixing. With increasing temperatures in early spring, phytoplankton export productivity grew, coupled with an increase of total mass flux and organic carbon, which might be associated to enhanced nutrient input from snowmelt runoff. The percentage of C37:4 shows some correspondence with observed seasonal changes in Lake Van’s stratification structure. We also evaluated the potential applicability of molecular temperature proxies derived from Archaea and haptophyte lipids. The use of the TEX86 proxy was precluded by low GDGT abundances. Estimated LCA temperatures were consistent with temperatures in the photic zone but no seasonality changes were observed despite the wide annual temperature range measured at Lake Van. 2011 Elsevier Ltd. All rights reserved.

The UK37′ index has proven to be a robust proxy to estimate past sea surface temperatures (SSTs) over a range of time scales, but like any other proxy, it has uncertainties. For instance, in reconstructions of the Last Glacial Maximum (LGM) in the northern North Atlantic, UK37′ indicates higher temperatures than those derived from foraminiferal proxies. Here we evaluate whether such warm glacial estimates are caused by the advection of reworked alkenones in ice-rafted debris (IRD) to deep-sea sediments. We have quantified both coccolith assemblages and alkenones in sediments from glaciogenic debris flows in the continental margins of the northern North Atlantic, and from a deep-sea core from the Reykjanes Ridge. Certain debris flow deposits in the North Atlantic were generated by the presence of massive ice-sheets in the past, and their associated ice streams. Such deposits are composed of the same materials that were present in the IRD at the time they were generated. We conclude that ice rafting from some locations was a transport pathway to the deep sea floor of reworked alkenones and pre-Quaternary coccolith species during glacial stages, but that not all of the IRD contained alkenones, even when reworked coccoliths were present. We speculate that the ratio of reworked coccoliths to alkenone concentration might be useful to infer whether significant reworked alkenone inputs from IRD did occur at a particular site in the glacial North Atlantic. We also observe that alkenones in some of the debris flows contain a colder signal than estimated for LGM sediments in the northern North Atlantic. This is also clear in the deep-sea core studied where the warmest intervals do not correspond to the intervals with large inputs of reworked coccoliths or IRD. We conclude that any possible bias to UK37′ estimates associated with reworked alkenones is not necessarily towards higher values, and that the high SST anomalies for the LGM are unlikely to be the result of a bias caused by IRD inputs.

New multiproxy marine data of the Eemian interglacial (MIS5e) from the Norwegian Sea manifest a cold event with near-glacial surface ocean summer temperatures (3–4 °C). This mid-Eemian cooling divided the otherwise relatively warm interglacial climate and was associated with widespread expansions of winter sea-ice and polar water masses due to changes in atmospheric circulation and ocean stability. While the data also verify a late rather than early last interglacial warm peak, which is in general disharmony with northern hemisphere insolation maximum and the regional climatic progression of the early Holocene, the cold event itself was likely instrumental for delaying the last interglacial climate development in the Polar North when compared with regions farther south. Such a ‘climatic decoupling’ of the Polar region may bear profound implications for the employment of Eemian conditions to help evaluate the present and future state of the Arctic cryosphere during a warming interglacial.

Birth of the Cool Over the past 4 million years or so, tropical sea surface temperatures have experienced a cooling trend (see the Perspective by Philander ). Herbert et al. (p. 1530 ) analyzed sea surface temperature records of the past 3.5 million years from low-latitude sites spanning the world's major ocean basins in order to determine the timing and magnitude of the cooling that has accompanied the intensification of Northern Hemisphere ice ages since the Pliocene. Martínez-Garcia et al. (p. 1550 ) found that the enigmatic eastern equatorial Pacific cold tongue, a feature one might not expect to find in such a warm region receiving so much sunlight, first appeared between 1.8 and 1.2 million years ago. Its appearance was probably in response to a general shrinking of the tropical warm water pool caused by general climate cooling driven by changes in Earth's orbit.

New micropaleontological, palynological, and geochemical results from a relatively shallow (∼ 500 m) sediment core (NS-14) in the south-eastern Aegean Sea provide a detailed picture of the regional expression of sapropel S1 formation in this sub-basin of the eastern Mediterranean Sea. Specifically, freshwater input during ∼ 10.6–10.0 ka BP has preceded the deposition of S1. Further decrease in surface water salinity is evidenced between 10.0 and 8.5 ka BP at the lower part of S1a, which in respect to S1b, is featured by warmer (∼ 19.5 °C) and more productive surface waters associated with dysoxic bottom conditions. A series of coolings detected within the S1 depositional interval, may be linked to outbursts of cold northerly air masses and relevant pulses in the deep-intermediate water ventilation that caused the S1 interruption between 7.9 and 7.3 ka BP and culminated during the deposition of S1b, with the decline of deep chlorophyll maximum (DCM) at ∼ 6.5 ka BP. The climate instability and the relevant absence of anoxia weakened the organic matter preservation in the shallow south-eastern Aegean margin during the S1 times. NS-14 record provides evidence for a distinct mid Holocene warm (up to ∼ 25 °C) and wet phase associated with the deposition of the sapropel-like layer SMH (Sapropel Mid Holocene), between 5.4 and 4.3 ka BP. The SMH layer could represent evidence of on-going, albeit weak, African monsoon forcing, only expressed at the south-eastern edge of the Aegean Sea. Its end is associated with the 4.2 ka BP Northern Hemisphere mega-drought event and the termination of the African Humid Period at 3.8 ka BP.

Sea surface temperatures (SSTs) and annual mean air temperatures (MATs) are estimated for the last 200 years from glycerol dialkyl glycerol tetraethers (GDGTs) proxies in a marine sediment core from the Skagerrak, off southern Norway. The reconstructed values compare well with annual SSTs and summer air temperatures obtained from composite regional instrumental records. The results provide further confidence in the application of proxies based on GDGTs to estimate past temperatures.

Long chain alkenones and photosynthetic pigment derivatives (chlorins) are commonly employed biomarkers in the reconstruction of past sea surface temperatures (SSTs) and organic carbon export from the surface to the deep ocean. For these studies, marine sediment samples are often available which have been air dried or freeze-dried, given that these are common methods for removing water from sediments prior to microfossil analysis. We compared both drying procedures by analysing samples from two sediment cores (ODP 1087, southeast Atlantic and M36/2-MC435, North Atlantic). We found that significant losses of chlorins (>25%) and alkenones (up to 75%) occur with air-drying relative to freeze-drying. Changes in the composition of pigments are also likely. The alkenone unsaturation index U 37 K ′ is sensitive to the drying method, showing an increase of up to 0.02 units relative to freeze-drying. However, this represents a maximum increase in SST estimates of only 0.5 °C. The tests suggest that freeze dried samples, rather than air dried sediments, should preferably be used to determine the sedimentary concentration of biomarkers. However, if chlorins are the main target compounds, wet extraction with acetone of sediment results in ca. 12% higher yield than the extraction of freeze-dried sediments with acetone/water.

The palaeoceanography of the northern Icelandic Shelf for the Holocene period was reconstructed from alkenone indices measured in core JR51-GC35. This contains a continuous record of Holocene sedimentation spanning 0 10.2 cal. kyr BP with a resolution of ~ 20 yr/cm. We have identified a general Holocene cooling trend that has superimposed millennial-scale oscillations of >2°C. Their timing is in close agreement with the timing of glacier advances in northern Iceland. For the later half of the Holocene, the alkenone-sea surface temperature (SST) record from JR51-GC35 correlates with proxy data for the strength of NADW formation recorded in cores south of Iceland. This is interpreted as evidence of a close connection existing between north Icelandic sea surface temperatures and the North Atlantic meridonal overturning circulation. The timing of the millennial-scale SST variability in our core off North Iceland is found to be out of phase, or anti-phased, with the SST variability of a record in the eastern Nordic Seas (MD952011). This suggests that the evolution of Holocene climate in the Nordic Seas was more complex than previously proposed; and it is likely to be caused by differential responses of the Irminger and Norwegian Currents and modulated by changes in atmospheric circulation analogous to the North Atlantic Oscillation.

The late Pliocene (Piacenzian) in the Mediterranean region was punctuated by short-lived episodes of widespread deposition of organic-rich sedimentary layers known as sapropels. The causes of their formation remain a long-standing debate in the science community, and require disentangling the roles of climatic/oceanographic processes that triggered higher primary productivity or enhanced organic matter preservation. The lack of data, especially of sea temperatures at sufficient temporal resolution, is one of the main challenges to solve this debate. Here, we present new organic geochemistry and micropaleontological data from the late Pliocene at Punta Grande/Punta Piccola sections (southwest Sicily) that allow untangling the mechanisms that favored the formation of two sapropel series (noted S and A) in the central Mediterranean area during this period. Sea surface (SSTs) and subsurface temperatures were estimated using three distinct organic geochemical proxies namely the alkenone unsaturation index ( U K ′ 37 ), the long-chain diol index (LDI) and the tetraether index (TEX86). Reconstructed SSTs are relatively stable throughout the late Pliocene and ∼4 °C higher than modern Mediterranean SSTs, which is consistent with the climatic conditions inferred for this period from paleoclimate modeling. An increase in SST is, however, recorded by U K ′ 37 and LDI proxies across each sapropel horizon, supporting that the two sapropel series S and A were formed during warmer climate conditions. The comparison of SST data with variations in accumulation rates of total organic carbon and lipid-biomarkers (alkenones, long-chain alkyl diols, archaeal and bacterial tetraethers), and with changes in calcareous nannofossil assemblages, indicates that the studied sapropels might have formed under different environmental conditions. The first series of sapropels (S), deposited between 3.1 and 2.8 Ma, is likely due to a better preservation of organic matter, induced by the development of a strong thermohaline stratification of the water column and to oxygen-depleted bottom waters. Higher terrestrial input that occurred between 3.1 and 2.8 Ma may interestingly explain the large discrepancies observed between TEX86 and U K ′ 37 -LDI temperature values during this period. The second series of sapropels (A), deposited between 2.7 and 2.6 Ma, is more likely due to enhanced primary productivity in a weakly-stratified water column.

The application of biomarker climate proxies to lacustrine settings requires a thorough understanding of those processes that give rise to and mediate in the burial and preservation of organic matter in the sediments. This information is to date missing for Lake Baikal in Central Asia. The biogeochemistry of the lake cannot be considered analogous to that in other lacustrine environments given its enormous size, depth, remote location and unique biology. The aim of this paper is to report on the main compound classes in the water column and sediments. As part of an ongoing evaluation of the inputs and fluxes of organic matter in Lake Baikal, we have found that there are significant differences in the inputs and preservation of organic matter between the North and South Basins. Both basins have dominant algal input and a contribution from the terrestrial plants of the lake edges and surrounding environments. However, the concentration of organic material in the sediment traps and sediments of the South Basin is much higher than that of the North Basin. In addition, the South Basin contains much higher proportions of the more labile material than does the North Basin. This is likely due to the South Basin being free of surface ice for a much longer time than the North, and in consequence, primary producers having an overall longer productive season. There is some evidence of microbial activity in the sediment traps and sediments from both the North and South Basins, but it does not appear to be more predominant in one basin than the other. It is probable that the differences in the composition and concentration of biomarkers in sediments between basins is due to the length of and the intensity of the productive season, which is in turn influenced by the climate.

Michal Kucera , Antoni Rosell-Mele, Ralph Schneider, Claire Waelbroeck, Mara Weinelt Department of Geology, Royal Holloway University of London, Egham TW20 0EX, UK ICREA and Institute of Environmental Science and Technology, Universitat Autonoma de Barcelona, 08193 Bellaterra, Catalonia, Spain Departement de Geologie et Oceanographie, UMR5805-EPOC, CNRS/Universite de Bordeaux1, 33405 Talence Cedex, France Laboratoire des Sciences du Climat et de l’ Environnement, Laboratoire mixte CNRS-CEA, F-91198 Gif sur Yvette, France Institute for Geosciences, University of Kiel, Olshausen Strasse 40, D-24098 Kiel, Germany

The thermohaline circulation is considered responsible for regulating climate variability at a global scale [Geochim. Cosmochim. Acta 53 (1989) 2465]. It has been argued that it can be disrupted by changes in surface ocean salinity in the sites of deepwater formation [J. Geophys. Res. 96 (1991) 16811]. However, to date reconstruction of sea surface salinity (SSS) has lacked the accuracy to identify the location, extent and amplitude of past meltwater events [Science 282 (1998) 61]. Recently, a measurement based on alkenones (%37:4) has been shown to be related to types of water masses and perhaps surface salinity in the Nordic Seas [Paleoceanography 13 (1998) 694; Terra Nova 10 (1998) 86]. Here, we present new data from surface sediments that further confirm that it is possible to obtain a linear correlation between %37:4 and salinity. Records of %37:4 from three nearby sediment cores in the NE Atlantic show equivalent profiles, with the highest values occurring in the Heinrich layers (HLs). We propose that downcore %37:4 data can be interpreted in terms of decreases in surface ocean salinity (freshening of surface waters) in response to incoming freshwater from sea-ice and/or icebergs to the core relation. The %37:4 data provides further indication of the locally lower surface ocean salinity that prevailed in the North Atlantic during the last glacial period, and particularly during the Heinrich episodes due to the influx of iceberg meltwater.

Geological evidence indicates that sediment reworking is common around the continental margins and abyssal depths of the Norwegian-Greenland Sea, a high-latitude setting with glacier-influenced margins. Detailed analysis of 22 cores up to 5 in long, placed in context by accompanying geophysical data including high resolution sub-bottom profiles, swath bathymetry and backscatter maps, indicates that reworking is variable and ranges from debris flows and turbidity currents, to bottom-current activity, as well as iceberg scouring. Reworking by debris flows appears to be restricted mainly to the main trough-mouth fans and sediment slides. Elsewhere, turbidity-current activity frequently dominates, although iceberg ploughing down to 600 in depth and current winnowing assume increasing significance on continental shelves. Reworking in the Norwegian-Greenland Sea reflects variations in ice-sheet dynamics that, in turn, influence the rate of sediment delivery and location of depocentres. Spatial variations in the style of reworking may also reflect the influence of continental slope gradient and bedrock geology on continental shelves. The widespread nature of sediment reworking has important implications for palaeoceanographic investigations in the region, as reworking can result in erosion and disturbance of the sediment column. It is estimated that less than 7% of material delivered to the Norwegian-Greenland Sea since the Late Weichselian is derived from hemipelagic and pelagic sedimentation. This problem is significant where continuous, high-resolution records of hemipelagic and pelagic sedimentation are required, and attempts are made to correlate with other high-resolution proxy records, such as ice cores, at sub-millennial scales. Bioturbation results in the smoothing of high-resolution records and imposes a maximum resolution for sediment-core time-slices of generally 400 years or more. In the Norwegian-Greenland Sea, areas of high sedimentation such as trough-mouth fans or contourite drifts are commonly associated with extensive reworking. Identification of reworking is particularly important where attempts are made to link records of iceberg-rafted debris to past ice-sheet dynamics, as bottom-current winnowing and mass-flow processes can increase the concentration of coarse-grained iceberg-rafted debris. Such localized accentuation of the iceberg-rafted debris signal may lead to erroneous palaeo-environmental interpretations. It is therefore critical that palaeoceanographic interpretations are firmly underpinned by an explicit sedimentological assessment of reworking.

A deep-sea sediment core underlying the Benguela upwelling system off southwest Africa provides a continuous time series of sea surface temperature (SST) for the past 4.5 million years. Our results indicate that temperatures in the region have declined by about 10°C since 3.2 million years ago. Records of paleoproductivity suggest that this cooling was associated with an increase in wind-driven upwelling tied to a shift from relatively stable global warmth during the mid-Pliocene to the high-amplitude glacial-interglacial cycles of the late Quaternary. These observations imply that Atlantic Ocean surface water circulation was radically different during the mid-Pliocene.

We compare alkenone unsaturation ratios measured on recent sediments from the Indian Ocean (20°N–45°S) with modern sea oceanographic parameters. For each of the core sites we estimated average seasonal cycles of sea surface temperature (SST) and salinity, which we then weighted with the seasonal productivity cycle derived from chlorophyll satellite imagery. The unsaturation index (U37K′) ranges from 0.2 to 1 and correlates with water temperature but not with salinity. TheU37K′versus SST relationship for Indian Ocean sediments (U37K′= 0.033 SST + 0.05) is similar to what has been observed for core tops from the Pacific and Atlantic oceans and the Black Sea. A global compilation for core tops givesU37K′= 0.031 T + 0.084 (R= 0.98), which is close to a previously reported calibration based on particulate organic matter from the water column. For temperatures between 24° and 29°C, however, the slope seems to decrease to about 0.02U37K′unit/°C. For Indian Ocean core tops, the ratios of total C37alkenones/total C38alkenones and the slope of theU37K′-SST relationship are similar to those previously observed for cultures ofEmiliania huxleyibut different from those previously published forGephyrocapsa oceanica.EitherE. huxleyiis a major producer of alkenones in the Indian Ocean or strains ofG. oceanicaliving in the northern Indian Ocean behave differently from the one cultured. In contrast with coccolithophorid assemblages, the ratios of C37alkenones to total C38alkenones lack clear geographic pattern in the Indian Ocean.

The so-called Heinrich layers, found in North Atlantic sediments, have been attributed to quasi-periodic discharges of armadas of icebergs originating from the Laurentide ice sheet. Using the distribution of biomarkers in a sediment core (BOFS 5K) we have attempted to provide more information about their origin and the effect of the Heinrich events on oceanic circulation and the climatic system. Estimates of sea surface temperature changes, via U3K7 measurements, show that the surface ocean underwent significant cooling during the events, probably as a result of incoming iceberg meltwater. The low sea temperatures and the rapid variation in the accumulation of chlorophyll-derived pigments suggest that the events provoked a major change in oceanographic conditions, which may have affected the thermohaline circulation in the Atlantic Ocean. Within the most recent layers we have also found vanadyl alkyl porphyrins and aryl isoprenoid hydrocarbons, diagenetic products of photosynthetic pig- ments (chlorophylls and carotenoids, respectively). These components are not expected to occur in late Quaternary sediments, so their presence demonstrates a contribution to the layers of ancient, organic rich sedimentary material of continental origin. The presence of vanadyl porphyrins and absence of their nickel counterparts, and the distributional features of the aryl isoprenoids (e.g., presence of the C40 reduced carotenoid isorenieratane of green sulphur bacterial origin) indicate the existence of anoxic conditions in the photic zone of a marine water column in the original depositional setting of the contributed ancient sediment. These results suggest that it should be possible, using a biomarker correla- tion approach, to determine the source area of the ancient sediment eroded by the icebergs. Copyright © 1997 Elsevier Science Ltd

A field calibration of the UK index with sea surface temperature is discussed, through analysis of an extensive suite of surface sediments ( n = 109) from the northeastern Atlantic (2°S–75°N). Values of U 37 K are compared with sea surface temperatures for overlying waters measured at different depths and seasons, to obtain a correlation suitable for palaeotemperature reconstructions. The best fit is obtained using surface (0 m) temperatures corresponding to caloric winter and autumn months. However, the annual average, spring and summer surface temperature equations also have high correlation coefficients, and are also appropriate for climatic studies. The results further validate the general applicability of the U 37 K as a climatic proxy, because the calibration equations are valid over a wide range of surface water temperatures (0–28°C) for different algal populations and are representative of the average contribution of alkenones to sediments, as found in sediment cores.

This study was designed to see if the intensity and location of upwelling in the Benguela Current Upwelling System off Namibia changed significantly during the last 70,000 years. Most of the analytical work focused on geochemical, micropalaeontological and stable isotopic analyses of a 6.5m long combined pilot and piston core, PGPC12, from 1017m on the continental slope close to Walvis Bay. The slope sediments are rich in organic matter. Most of it is thought to represent deposition beneath a productive shelf edge upwelling system, but some is supplied by downslope nearbottom flow of material probably resuspended on the outer continental shelf. Temporal changes in upwelling intensity as represented by fluctuations in the accumulation of organic matter do not show the simple ‘classical’ pattern of less upwelling and lower productivity in interglacials and more upwelling and higher productivity in glacials, but instead show a pattern of higher frequency fluctuations. The broad changes in organic carbon accumulation reach maxima at times when the earth-sun distance was greatest, indicating that this accumulation responded to changes in the precession index; at these times monsoons would have been weakest and Trade Winds strongest. Maximum accumulation of organic matter on the slope occurred in the last interstadial (isotope stage 3), and coincided with coldest sea surface temperatures as recorded by alkenone data (U k 37 ), and by nannofossil assemblages. It is attributed largely to increased productivity in situ , rather than the lateral supply of material eroded from older organic rich deposits exposed by the lowering of sealevel at that time. The enhanced productivity is attributed to a strengthening of upwelling-favourable winds in this area in response to the minimal solar insolation typical of this period. Diatoms generally are not abundant in these sediments, so appear to be unreliable indicators of productivity over the continental slope. When sealevel was lowest (isotope stages 2 and 4) organic matter previously deposited on the continental shelf was eroded and dumped on the continental slope; this reworked material constitutes up to 43% of the flux of organic matter to the slope at these times. This process did not affect the slope in stage 3, when sealevel fell by only 50m. The accumulation of terrigenous material was highest in stages 2 and 4. The available data suggest that the terrigenous influx at those times was primarily aeolian. We interpret this to mean that more of the winds then came from the east (‘Berg’ winds), bringing an influx of aeolian dust from the hinterland; these easterlies were less favourable for upwelling than were the more southerly Trade Winds that dominated during stage 3. Carbonate accumulation was least in stages 2 and 4, largely in response to dissolution induced by CO 2 -rich bottom waters.

[1] In spite of the important role played by the Southern Ocean in global climate, the few existing paleoceanographic records in the east Pacific sector do not extend beyond one glacial-interglacial cycle, hindering circumpolar comparison of past sea surface temperature (SST) evolution in the Southern Ocean. Here we present three alkenone-based Pleistocene SST records from the subantarctic and subtropical Pacific. We use a regional core top calibration data set to constrain the choice of calibrations for paleo SST estimation. Our core top data confirm that the alkenone-based UK37 and UK′37values correlate linearly with the SST, in a similar fashion as the most commonly used laboratory culture-based calibrations even at low temperatures (down to ∼1°C), rendering these calibrations appropriate for application in the subantarctic Pacific. However, these alkenone indices yield diverging temporal trends in the Pleistocene SST records. On the basis of the better agreement withδ18O records and other SST records in the subantarctic Southern Ocean, we propose that the UK37 is a better index for SST reconstruction in this region than the more commonly used UK′37 index. The UK37-derived SST records suggest glacial cooling of ∼8°C and ∼4°C in the subantarctic and subtropical Pacific, respectively. Such extent of subantarctic glacial cooling is comparable to that in other sectors of the Southern Ocean, indicating a uniform circumpolar cooling during the Pleistocene. Furthermore, our SST records also imply massive equatorward migrations of the Antarctic Circumpolar Current (ACC) frontal systems and an enhanced transport of ACC water to lower latitudes during glacials by the Peru-Chile Current.

The abundances of long-chain alkenones and alkyl alkenoates are reported for a suite of surface sediments from the NE Atlantic (0°–75°N). Despite the differing conditions of their biologies and differing extents of water column degradation and of early diagenesis in the sediments, the degree of unsaturation of the alkenones (e.g. the U 37 k parameter), and the relative proportion of alkyl alkenoates to alkenones display linear correlations with average temperature for surface waters warmer than 5°C. There is evidence that the chain-length distribution of the alkenones and of the ratio between the ethyl and methyl C 38 alkenones are species dependent, which would enable their use as chemotaxonomic tools.

Dust has the potential to modify global climate by influencing the radiative balance of the atmosphere and by supplying iron and other essential limiting micronutrients to the ocean. Indeed, dust supply to the Southern Ocean increases during ice ages, and 'iron fertilization' of the subantarctic zone may have contributed up to 40 parts per million by volume (p.p.m.v.) of the decrease (80-100 p.p.m.v.) in atmospheric carbon dioxide observed during late Pleistocene glacial cycles. So far, however, the magnitude of Southern Ocean dust deposition in earlier times and its role in the development and evolution of Pleistocene glacial cycles have remained unclear. Here we report a high-resolution record of dust and iron supply to the Southern Ocean over the past four million years, derived from the analysis of marine sediments from ODP Site 1090, located in the Atlantic sector of the subantarctic zone. The close correspondence of our dust and iron deposition records with Antarctic ice core reconstructions of dust flux covering the past 800,000 years (refs 8, 9) indicates that both of these archives record large-scale deposition changes that should apply to most of the Southern Ocean, validating previous interpretations of the ice core data. The extension of the record beyond the interval covered by the Antarctic ice cores reveals that, in contrast to the relatively gradual intensification of glacial cycles over the past three million years, Southern Ocean dust and iron flux rose sharply at the Mid-Pleistocene climatic transition around 1.25 million years ago. This finding complements previous observations over late Pleistocene glacial cycles, providing new evidence of a tight connection between high dust input to the Southern Ocean and the emergence of the deep glaciations that characterize the past one million years of Earth history.

Reconstruction of late Quaternary sea level history in areas of glacioisostatic uplift often relies on sediment archives from coastal isolation basins, natural coastal rock depressions previously isolated from or connected to the sea at different times. Proxy indicators for marine, brackish, or lacustrine conditions combined with precise dating can constrain the time when the sea crossed the sill threshold and isolated (or connected) the basin. The utility of isolation basins in investigations of sea level change is well known, but investigations have been mostly limited to microfossil proxies, the application of which can be limited by preservation and nonanalog problems. Here we investigate the potential of long-chain alkenones, alkenoates, and bulk organic parameters (TOC, Corg/N) for reconstructing past sea level changes in isolation basins in NW Scotland. We analyze organic biomarkers and bulk parameters from both modern basins (at different stages of isolation from the sea) and fossil basins (with sea level histories reconstructed from established proxies). Logit regression analysis was employed to find which of the biomarker metrics or bulk organic measurements could reliably characterize the sediment samples in terms of a marine/brackish or isolated/lacustrine origin. The results suggested a good efficiency for the alkenone index %C37:4 at predicting the depositional origin of the sediments. This study suggests that alkenones could be used as a novel proxy for sea level change in fossil isolation basins especially when microfossil preservation is poor.

Results from multiple model simulations are used to understand the tropical sea surface temperature (SST) response to the reduced greenhouse gas concentrations and large continental ice sheets of the last glacial maximum (LGM). We present LGM simulations from the Paleoclimate Modelling Intercomparison Project, Phase 2 (PMIP2) and compare these simulations to proxy data collated and harmonized within the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface Project (MARGO). Five atmosphere–ocean coupled climate models (AOGCMs) and one coupled model of intermediate complexity have PMIP2 ocean results available for LGM. The models give a range of tropical (defined for this paper as 15°S–15°N) SST cooling of 1.0–2.4°C, comparable to the MARGO estimate of annual cooling of 1.7 ± 1°C. The models simulate greater SST cooling in the tropical Atlantic than tropical Pacific, but interbasin and intrabasin variations of cooling are much smaller than those found in the MARGO reconstruction. The simulated tropical coolings are relatively insensitive to season, a feature also present in the MARGO transferred-based estimates calculated from planktonic foraminiferal assemblages for the Indian and Pacific Oceans. These assemblages indicate seasonality in cooling in the Atlantic basin, with greater cooling in northern summer than northern winter, not captured by the model simulations. Biases in the simulations of the tropical upwelling and thermocline found in the preindustrial control simulations remain for the LGM simulations and are partly responsible for the more homogeneous spatial and temporal LGM tropical cooling simulated by the models. The PMIP2 LGM simulations give estimates for the climate sensitivity parameter of 0.67°–0.83°C per Wm−2, which translates to equilibrium climate sensitivity for doubling of atmospheric CO2 of 2.6–3.1°C.

Recently, two new proxies based on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) were proposed, i.e., the TEX86 proxy for sea surface temperature reconstructions and the BIT index for reconstructing soil organic matter input to the ocean. In this study, fifteen laboratories participated in a round robin study of two sediment extracts with a range of TEX86 and BIT values to test the analytical reproducibility and repeatability in analyzing these proxies. For TEX86 the repeatability, indicating intralaboratory variation, was 0.028 and 0.017 for the two sediment extracts or +/-1-2°C when translated to temperature. The reproducibility, indicating among-laboratory variation, of TEX86 measurements was substantially higher, i.e., 0.050 and 0.067 or +/-3-4°C when translated to temperature. The latter values are higher than those obtained in round robin studies of Mg/Ca and U37 k' paleothermometers, suggesting the need to primarily improve compatibility between labs. The repeatability of BIT measurements for the sediment with substantial amounts of soil organic matter input was relatively small, 0.029, but reproducibility was large, 0.410. This large variance could not be attributed to specific equipment used or a particular data treatment. We suggest that this may be caused by the large difference in the molecular weight in the GDGTs used in the BIT index, i.e., crenarchaeol versus the branched GDGTs. Potentially, this difference gives rise to variable responses in the different mass spectrometers used. Calibration using authentic standards is needed to establish compatibility between labs performing BIT measurements.

Received 3 July 2008; revised 9 October 2008; accepted 27 October 2008; published 14 February 2009. [1] Paleoclimatic reconstructions have provided a unique data set to test the sensitivity of climate system to changes in atmospheric CO2 concentrations. However, the mechanisms behind glacial/interglacial (G/IG) variations in atmospheric CO2 concentrations observed in the Antarctic ice cores are still not fully understood. Here we present a new multiproxy data set of sea surface temperatures (SST), dust and iron supply, and marine export productivity, from the marine sediment core PS2489-2/ODP Site 1090 located in the subantarctic Atlantic, that allow us to evaluate various hypotheses on the role of the Southern Ocean (SO) in modulating atmospheric CO2 concentrations back to 1.1 Ma. We show that Antarctic atmospheric temperatures are closely linked to changes in SO surface temperatures over the last 800 ka and use this to synchronize the timescales of our marine and the European Project for Ice Coring in Antarctica (EPICA) Dome C (EDC) records. The close correlation observed between iron inputs and marine export production over the entire interval implies that the process of iron fertilization of marine biota has been a recurrent process operating in the subantarctic region over the G/IG cycles of the last 1.1 Ma. However, our data suggest that marine productivity can only explain a fraction of atmospheric CO2 changes (up to around 40‐50 ppmv), occurring at glacial maxima in each glacial stage. In this sense, the good correlation of our SST record to the EDC temperature reconstruction suggests that the initial glacial CO2 decrease, as well as the change in the amplitude of the CO2 cycles observed around 400 ka, was most likely driven by physical processes, possibly related to changes in Antarctic sea ice extent, surface water stratification, and westerly winds position.

[1] Past surface ocean circulation changes associated with the mid-Pleistocene transition, 0.9–0.6 Ma, were reconstructed in the northern North Atlantic (ODP 983) and the northwest Pacific (ODP 882), using proxies for subarctic/subpolar water mass distributions (%C37:4 alkenone) and sea surface temperature (U37 ). Both sites experienced a secular expansion of subarctic waters from � 1.15 Ma, spanning both glacial and interglacial intervals. After 0.9 Ma, low %C37:4 at Site 983 records a northward retreat of subarctic waters during interglacials in the Atlantic, while continued high glacial %C37:4 indicate extensive subarctic waters during glacial maxima associated with the development of the larger late Pleistocene ice sheets. In contrast, a secular decline in %C37:4 occurred at Site 882 from 0.9 to 0.5 Ma, marking a more gradual retreat of subarctic conditions in the Pacific. It is proposed that the expansion of subarctic waters between 1.15 and 0.9 Ma exerted negative feedbacks to the moisture supply to the ice sheet source regions and may account for the apparent delayed ice sheet response to atmosphere-ocean circulation changes associated with the mid-Pleistocene transition that began as early as 1.2 Ma.

Within the frame of the multiproxy approach for the reconstruction of the glacial ocean (MARGO) project, sea-surface conditions of the Last Glacial Maximum (LGM, 23–19 ka) were reconstructed using different proxies, which were calibrated to a standardized modern hydrography. In the North Atlantic, the revised LGM MARGO data set provides a comprehensive coverage, including the Nordic Seas. The data set includes reconstruction based on planktonic foraminifer assemblages, dinocyst assemblages, alkenone coccolithophorid biomarkers and Mg/Ca ratios in planktonic foraminifers. Several hydrological features of the LGM North Atlantic can be identified, that are supported by all four proxies. They include an extensive perennial sea-ice cover along the eastern Canadian and Greenland margins, seasonally ice-free central North Atlantic and Norwegian Sea, and the winter sea-ice limit being located at about 55°N. All proxies also suggest significantly colder than modern sea-surface conditions in the southeastern part of the North Atlantic and a more zonal temperature pattern than at present, with a steep SST gradient of the order of 10 °C between 40°N and 45°N. However, in the Nordic Seas large discrepancies remain, well above the level of uncertainty of the SST reconstructions. These discrepancies might be related to stratification of the upper water mass linked to large annual amplitude of temperature with contrasting winter and summer conditions, high interannual-interdecadal variability, or taphonomic processes affecting some of the proxies. The average LGM SST in the Nordic Seas cannot at present be assessed with confidence. However, the existing evidence suggests that highly variable sea-surface conditions and at least occasional advection of North Atlantic waters may have characterised the glacial Nordic Seas.

[1] Samples of filtered particulate organic matter (POM) were obtained during the summers of 1999 and 2000 from the surface waters of the Nordic seas to monitor the spatial distribution of long-chain alkenones. The aim of the study was to appraise existing alkenone-based climatic proxies in northern high latitudes. Unusually high percentages of the tetraunsaturated alkenone were measured in the polar waters of the East Greenland Current, with C37:4 of up to 77% in 80% of sea-ice cover. Values of percent C37:4 across the Nordic seas showed a strong association with water mass type. Analysis of coccoliths in filters indicated that calcified Emiliania huxleyi could not be discounted as the biological precursor of alkenones in all the water masses. A combined data set of 69 samples of POM revealed a stronger correlation of percent C37:4 to sea surface salinity (SSS; R2 = 0.72) than to sea surface temperature (SST; R2 = 0.50). Values of percent C37:4 in sea surface POM were much higher than those in surficial sediments of the northern North Atlantic. To explain the discrepancy in sedimentary and surface water column percent C37:4, we propose that the alkenone contents in surface sediments underlying arctic and polar waters are a combination of autochthonous and allochthonous inputs of alkenones. Our results show that percent C37:4 can be used to reconstruct the relative extension of arctic/polar water masses in the North Atlantic. However, the results prevent confirmation of percent C37:4 as a paleo-SSS proxy in the Nordic seas, given its multivariate nature in our data set and the decoupling between its range of values in surface waters and sediments.