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International audience; Paleoproductivity is reconstructed across a Mediterranean benchmark record, the Early/Middle Pleistocene Montalbano Jonico section, cropping out in southern Italy. High-resolution coccolithophore and alkenone data (C37 and C37:2/C38:2 ratio) were collected in order to extend the data set on Mediterranean paleoproductivity pattern and forcing mechanisms. The multi-proxy record indicates low productivity during glacial and stadial phases and enhanced productivity during interglacial and interstadials. Increased surface water turbidity, cold-water temperature and polar-subpolar low salinity water incursion appear as the dominant controls for low productivity during Marine Isotope Stage (MIS) 20. Enhanced productivity during MIS 19c was sustained by warmer surface waters, coupled with a seasonal precipitation regime, providing higher nutrient availability. Productivity increases during interstadials with respect to stadials, in relation with enhanced land-derived nutrient input through river discharge during wetter winters. The productivity scenario we propose is similar to those reconstructed from deep-sea records in the central and western Mediterranean during Dansgaard-Oeschger oscillations over the last 70 ka. This indicates that similar forcing mechanisms acted on productivity dynamics on a regional scale over different times. We suggest that migration of the westerly wind system over the Mediterranean and the polar water inflow influenced productivity on a regional scale. The acquired data set provides new evidences on the environmental significance of the C37:2/C38:2 ratio and on its relation with surface water productivity.

This study shows the first Mediterranean high-resolution record of alkenone-derived sea surface temperature (SST) in the marine sediments outcropping at the Ideale section (IS) (southern Italy, central Mediterranean) from late marine isotope stage (MIS) 20 - through early MIS 18. The SST pattern evidences glacial-interglacial up to submillennial-scale temperature variation, with lower values (~13 °C) in late MIS 20 and substage 19b, and higher values (up to 21 °C) in MIS 19c and in the interstadials of MIS 19a. The SST data are combined with the new calcareous plankton analysis and the available, chronologically well-constrained carbon and oxygen isotope records in the IS. The multi-proxy approach, together with the location of the IS near the Italian coasts, the lower circalittoral-upper bathyal depositional setting, and high sedimentation rate allow to document long-and short-term paleoenvironmental modifications (sea level, rainfall, inorganic/organic/fresh water input to the basin), as a response to regional and global climate changes. The combined proxies reveal the occurrence of a terminal stadial event in late MIS 20 (here Med-HTIX), and warm-cold episodes (here Med-BATIX and Med-YDTIX) during Termination IX (TIX), which recall those that occurred through the last termination (TI). During these periods and the following ghost sapropel layer (insolation cycle 74, 784 ka) in the early MIS 19, high frequency internal changes are synchronously recorded by all proxies. The substage MIS 19c is warm but quite unstable, with several episodes of paleoenvironmental changes, associated with fluctuating tropical-subtropical water inflow through the Gibraltar Strait, variations of the cyclonic regime in the Ionian basin, and the southward shift of westerly winds and winter precipitation over southern Europe and Mediterranean basin. Three high-amplitude millennial-scale oscillations in the patterns of SST and calcareous plankton key taxa during MIS 19a are interpreted as linked to changes in temperature as well as in salinity due to periodical water column stratification and mixing. The main processes involved in the climate variability include changes in oceanographic exchanges through the Gibraltar Strait during modulations of Atlantic meridional overturning circulation and/or variations in atmospheric dynamics related to the influence of westerly and polar winds acting in the paleo-Ionian basin. A strong climate teleconnection between the North Atlantic and Mediterranean is discussed, and a prominent role of atmospheric processes in the central Mediterranean is evidenced by comparing data sets at the IS with Italian and extra-Mediterranean marine and terrestrial records.

The warm Pliocene serves as an analogue for predicted warming over the next century. However, large uncertainties exist for atmospheric circulation and land surface conditions during the Pliocene. Dust transported by wind to locations of accumulation (terrestrial or marine) can provide a record of wind intensity and/or direction. Few dust flux records spanning the Plio-Pleistocene exist. As such, there is ample opportunity to use marine sediments to reconstruct changes in atmospheric conditions during a warmer-than-present world, as well as across the onset/intensification of Northern Hemisphere Glaciation (NHG). During this time, East Asia’s interior, the second largest source of mineral dust today, experienced aridification, occurring alongside a major reorganization of the subarctic North Pacific circulation which led to stratification of the surface ocean. Here, we present two North Pacific marine sediment records of extraterrestrial (ET) 3He-derived terrigenous dust flux proxies (4HeTerr and Th), along with a record of multiple paleoproductivity proxies (Baxs, Opal, and C37Total) for the period spanning ~2.5-4.5 Ma. Our results show that dust flux to the western North Pacific was relatively low and constant through the Pliocene up until ~2.7 Ma, with minor peaks during cooler phases from ~2.9-3.1 Ma. At ~2.7 Ma, concurrent with the intensification of NHG and formation of a permanent halocline cap in the subarctic North Pacific, dust fluxes increase dramatically. The central North Pacific record shows a less drastic shift in dust, but generally displays higher fluxes after ~3 Ma. Dust fluxes in East Asia and the North Pacific are consistent during this time interval, as are global dust fluxes from the North Atlantic, South Atlantic and North Pacific. Western North Pacific dust, SST, and paleoproductivity records point to northward-shifted and weakened Northern Hemisphere westerlies during the warm Pliocene, with evidence for strengthening and southward movement of the westerlies during glacials after ~2.7 Ma. Changes in both winds and dust production mechanisms are likely working in tandem to produce the coherent global dust signals.

Alkenone mass accumulation rates (MARs) provide a proxy for export productivity in the northwestern Pacific (Ocean Drilling Program Site 1208) spanning the late Pliocene through early Pleistocene (3.0-1.8 Ma). We investigate changes in productivity associated with global cooling during the onset and expansion of Northern Hemisphere glaciation (NHG). Alkenone MARs vary on obliquity time-scales throughout, but the amplitude increases at 2.75 Ma concurrent with the intensification of NHG and cooling of the sea surface by 3 °C. The obliquity-scale variations in alkenone MARs parallel shipboard measurements of sediment color reflectance (%) with higher MARs significantly correlated (>95%) with darker (opal-rich) intervals. Variations in both lead benthic foraminiferal δ18O values by 1.5-2 kyr, suggesting that export productivity may be a contributing factor, rather than a response, to the extent of continental glaciation. The biological pump is therefore a plausible mechanism for transferring atmospheric CO2 into the deep ocean during the onset of NHG and the ensuing obliquity dominated climate regime. Obliquity-scale correlation between productivity and magnetic susceptibility is consistent with a link via westerly winds delivering terrigenous sediments and mixing the upper water column. Alkenone MARs also contain a ~400 kyr modulation. Because this periodicity is a multiple of the residence time of carbon in the ocean, it may reflect inputs of new nutrients associated with eccentricity forced changes in the terrestrial biosphere and weathering. We ascribe these findings to interactions between the East Asian winter monsoon and productivity in the North Pacific Ocean, perhaps contributing to Plio-Pleistocene climate change.

It is difficult to untangle the mixed influences of high- and low-latitude climate forcing in the eastern equatorial Pacific (EEP). Here we test the hypothesis that the Southern Ocean drove change in the EEP via subsurface intermediate waters during the last deglaciation. We use the δ18O signature of benthic foraminifera to reconstruct water density changes during the last 25 kyr at three intermediate water depths (370 m, 600 m, and 1000 m) in the EEP. Carbonate δ18O records a combined signature of temperature and salinity and is therefore more closely related to density than temperature or salinity alone. We find that benthic foraminiferal δ18O values decreased first in the subsurface, simultaneously with rising temperatures over Antarctica, and propagated up to the surface within ~3 kyr. The early subsurface response initiated a rapid decrease in density stratification over the upper water column as indicated by reduced δ18O gradients between surface and intermediate depths. Stratification of the upper water column remained low through the termination, with stratification minima reached during Heinrich Stadial 1 and the Younger Dryas (YD), synchronous with the two-part deglacial rise in atmospheric CO2. Centennial-scale shifts toward heavier δ18O signatures at 370 and 600 m during the YD indicate short-lived shifts in the Subantarctic Mode Water/Antarctic Intermediate Water boundary to shallower intermediate depths. We suggest that decreased density gradients during the deglaciation accelerated vertical mixing across the EEP, and potentially the entire South Pacific subtropical gyre, which enhanced CO2 delivery from depth to the surface ocean and atmosphere.

We present a composite time series of Mediterranean sea surface temperature (SST) and marine biomarker accumulation for the time span from 3.5 to 1.525 Ma, based on alkenone unsaturation and concentration from hemipelagic sediments outcropping in southern Italy and Sicily. Paleotemperature data define three regimes: a late Pliocene climate on average 4 – 5 ° C warmer than modern, a latest Pliocene to early Pleistocene onset of 41 kyr cycles, and a major increase in the range of glacial–interglacial temperature change at ∼ 1.84 Ma that shortly precedes the former definition of the Plio-Pleistocene boundary in the Crotone sequence. Pliocene sea surface temperature (SST) cycles are dominated by precession, with a ∼ 1.5 ° C range. Obliquity-related rhythms influence SST significantly shortly after ∼ 2.8 Ma (equivalent to MIS G10) and dominate after ∼ 2.51 Ma (equivalent to MIS 100). However, little, if any, long-term cooling occurred on an interglacial basis until after ∼ 1.85 Ma . Alkenone concentrations provide a good proxy for the accumulation of marine organic matter, and primarily reflect regional hydrology. Organic sedimentation, including the formation of layers highly enriched in organic matter (“sapropels”) was paced throughout by precessional variations despite changes in both average regional temperature, and the shift in temperature variance to the 41 kyr obliquity cycle in the latest Pliocene and early Pleistocene. Our reconstruction therefore highlights the intermingling of both hemispheric-wide changes in temperature and regional variations in the hydrological cycle that combined to force major evolutionary changes in the fauna and flora of northern Africa and the southern Mediterranean in late Pliocene to mid-Pleistocene time.

[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.

A high-resolution grayscale series of the pelagic Fucoid Marls (Piobbico core, central Italy) shows strong, pervasive lithological rhythms throughout the Aptian interval. A hierarchy of centimeter- to meter-scale cycles characterizes the rhythms; when calibrating ~1 m cycles to Earth’s 405 k.y. orbital eccentricity cycle, these rhythms correspond to the periods of the eccentricity, obliquity, and precession index. Tuning to orbital eccentricity cycles provides a high-resolution time scale for the Aptian. Correlation to the Cismon core (northern Italy) extends the tuning to the Aptian-Barremian boundary. The tuning indicates a minimum duration of 13.42 m.y. for the Aptian Stage, where previous estimates range from 6.4 to 13.8 m.y. The combined Aptian–Albian astronomical tuning of the entire 77-m-long Piobbico core (and part of the Cismon core) provides a 25.85-m.y.-long astronomically calibrated time scale for Earth history.

The alkenone organic paleotemperature proxy has been used with great success to generate sea-surface temperature (SST) records from diverse areas of the world ocean. To date, however, this methodology has been applied almost exclusively to unconsolidated marine sediments; the fidelity of alkenone preservation in lithified, uplifted marine sequences exposed on land has not been explicitly investigated. Such sedimentary sequences have been buried at depths of hundreds of meters to kilometers before uplift, and in addition have been exposed to oxic pore fluids and subaerial weathering upon ascent to the surface, any of which may cause alteration of biomarker compounds. In this study, we compare alkenone SST estimates from the Vrica land section in Crotone, Italy, with alkenone SST estimates derived from coeval marine sediments retrieved at Ocean Drilling Program Site 964, which is ~300 km offshore of the Vrica location. We find that SST records from the land and marine sections are well correlated, showing similar mean values, cyclicity, and amplitudes. Although results from different sedimentary sequences might depend on the lithologies, burial depths, and weathering histories of individual outcrops, this study suggests that it will be possible to reconstruct quantitative SST histories from marine strata exposed on land.

The early Aptian Oceanic Anoxic Event (OAE1a, 120 Ma) represents a geologically brief time interval in the mid-Cretaceous greenhouse world that is characterized by increased organic carbon accumulation in marine sediments, sudden biotic changes, and abrupt carbon-isotope excursions indicative of significant perturbations to global carbon cycling. The brevity of these drastic environmental changes (

The large 100-kyr cycles evident in most late-Pleistocene (0–0.6 Ma) paleoclimatic records still lack a satisfactory explanation. Previous studies of the nature of the transition from the early Pleistocene (1.2–1.8 Ma) 41-kyr-dominated climate regime to the 100-kyr world have been based almost exclusively on benthic foraminiferal oxygen isotopic (δ18O) data. It is generally accepted that the late Pleistocene 100-kyr cycles represent a newly evolved sensitivity to eccentricity/precession, superimposed on an earlier, and largely constant, response to obliquity and precession forcing. However, orbitally-resolved Pleistocene sea surface temperature (SST) records from a variety of oceanic regions paint a rather different picture of the global climate transition across the mid-Pleistocene transition (MPT, 0.6–1.2 Ma). Reanalysis of these SST records shows that: (1) an early onset of strong 100-kyr-like cycles in two low-frequency bands (∼ 120–145 kyr and ∼ 60–80 kyr), derived from the bundling of two/three obliquity cycles into grand cycles (obliquity subharmonics), occurred in tropical SST records during the early Pleistocene, (2) these two early Pleistocene periods converge into the late-Pleistocene 100-kyr period in tropical SST records, (3) the dominance of 100-kyr SST power in the late Pleistocene coincides with a dramatic decline in the 41-kyr SST power, and (4) the correlation of timing of glacial terminations with eccentricity/precession variation could well extend back into the early Pleistocene. We demonstrate that most of these features also occur in δ18O records, but in a much more subtle manner. These features could be explained in two plausible ways: a shift in climate sensitivity from obliquity to eccentricity/precession (a modified version of the conventional view) or an increasingly nonlinear response to orbital obliquity across the MPT. However, our examination of the development of ∼100-kyr cycles favors an obliquity bundling mechanism to form late Pleistocene 100-kyr cycles. We therefore suggest that the late Pleistocene 100-kyr climatic cycles are likely a nonlinear response to orbital obliquity, although the timing of late Pleistocene 100-kyr climatic cycles and their early forms appears to be paced by eccentricity/precession.

We have investigated glacial-interglacial differences in sea surface temperature (SST) near Hawaii using two relatively high deposition rate, shallow-water piston cores collected near Oahu, Hawaii. Modern hydrographic data show that local surface water temperatures are broadly consistent with the regional pattern of SSTs in the southern subtropical North Pacific. Past SSTs were estimated on the basis of three independently measured parameters: (1) U37k′ values of alkenones, (2) δ18O of Globigerinoides ruber, and (3) assemblages of planktonic foraminifera using the modern analog technique (MAT). The two cores yield similar SST records, and if differences in the ecology of foraminifera and coccolithophores are considered, the three different approaches to estimating SSTs yield consistent results. U37k′-based temperatures, which may represent winter values at this location, were ∼2.5°C colder during the Last Glacial Maximum than today, which is consistent with the February MAT estimates. The δ18O-based temperature estimates, likely biased toward summer temperatures, indicate that the glacial SSTs were at least 1°C cooler than today, which is comparable to the results of MAT August estimates.

Drill sites recovered from the South Atlantic by the Deep Sea Drilling Project provide excellent magnetostratigraphies for the time–interval of polarity chrons C33r through C28n (83 to 62.5 Ma: Cande and Kent time–scale). Cyclic patterns of carbonate sedimentation, with a mean repeat time of ca . 20 ka, can be correlated between sites. The cycles show the full hierarchy of eccentricity amplitude modulations that would be expected of precessional orbital forcing. A significant modulation component exists at ca . 400 ka, which in all likelihood matches the modern 404 ka repeat time for eccentricity. This modulation pattern may provide the ‘tuning fork’ for tying cyclical sedimentation to the most stable astronomical eccentricity period. A ca . 2.5 Ma modulation also emerges that seems surprisingly similar to the modern long–wavelength eccentricity modulation of precessional amplitude, which should average 2.425 Ma. Results of ‘cyclochronology’ of polarity chrons C29n to C31n suggest that the Cande and Kent time–scale will need to be revised to allow for a more gentle change in South Atlantic sea–floor spreading than modelled by those authors.

Alkenone unsaturation indices (Uk'37) of marine sediment could prove particularly useful on organic-rich continental margins where carbonate dissolution hampers the use of other paleoclimatic proxies [McCaffrey et al., 1990; Kennedy and Brassell, 1992]. Forty core top samples of Recent sediment from a latitudinal transect (23°–40°N) along the California margin yield Uk'37 values that correlate linearly with modern mean annual sea surface temperatures (SSTs) in the range of 12°–23°C. Reproducibility of the unsaturation value in closely spaced cores is near analytical error. Uk'37 data define a relationship to temperature nearly identical to the Prahl et al. [1988] laboratory cultures of Emiliania huxleyi. The close agreement is particularly significant in light of the nannofossil composition of the sediments, where the abundance of the coccolith taxon Gephyrocapsa oceanica (known to synthesize alkenones) equals or exceeds that of E. huxleyi. Comparison with seasonal temperature variations at different depths indicates that little if any alkenone production occurs at depths >30 m along the continental margin (water depths

We have compiled 19 records from marine carbonate cores in which the Matuyama-Brunhes boundary (MBB) has been reasonably well constrained within the astronomically forced stratigraphic framework using oxygen isotopes. By correlation of the δ18O data to a timescale based on astronomical forcing, we estimate astronomical ages for each of the MBB horizons. In all but one record the MBB occurs within Stage 19. Most magnetostratigraphic sections in Asian Loess place the MBB within a loess interval. Since loess deposition is presumed to be associated with glacial intervals, loess horizons should correspond to even-numbered oxygen isotope stages. A glacial age for the MBB is at odds with the results presented here, which firmly place the MBB within interglacial Stage 19. Inconsistency among the many loess sections and between the loess and the marine records suggests that the magnetic interpretation of loess sections may be more complicated than hitherto supposed. The mean of the Stage 19 age estimates for the MBB is 777.9 ± 1.8 (N = 18). Inclusion of the single Stage 20 age results in a mean of 778.8 ± 2.5 (N = 19). The astronomical age estimate of the MBB compares favorably with an (unweighted) mean of 778.2 ± 3.5 (N = 10) from a compilation of 40Ar/39Ar results of transitional lava flows. Combining the two independent data sets yields a grand mean of 778.0 ± 1.7 (N = 28). The new compilation shows virtually no trend in placement of the MBB within isotope Stage 19 as a function of sediment accumulation rate. We interpret this to mean that the average depth of remanence acquisition is within a few centimeters of the sediment-water interface. Separating the cores into two geographic regions (an Indo-Pacific-Caribbean [IPC] Group and an Atlantic Group) results in a significant difference in the position of the mid-point of the reversal with respect to the astronomical time scale. The data presented here suggest a difference of several thousand years between the two regions. This observation could be caused by systematic differences between the two regions in sedimentation rate within the interval of interest, systematic differences in remanence acquisition, or by genuine differences in the timing of the directional changes between the two regions.

The magnetic properties of the sediments (“rock magnetics”) at DSDP Site 522 in the South Atlantic exhibit clear differences between the latest Eocene and earliest Oligocene. Based on low temperature behavior of saturation remanence and hysteresis loops, we attribute the difference to a slightly greater proportion of the finest grained, so-called “superparamagnetic” magnetite in the Eocene sediments. We believe that the lower proportion of very fine-grained magnetite in the Oligocene sediments is a result of incipient reduction diagenesis caused by increased productivity and hence increased labile organic carbon transport to the sediments due to an early Oligocene increase in thermohaline circulation. The Eocene-to-Oligocene transition at Site 522 is also expressed by changes in microfossil assemblages, increased carbonate content, decreased insoluble residue, and decreased foraminiferal shell fragmentation. The increase in carbonate is synchronous with and parallels a change in the ratio of two of the rock magnetic parameters, a ratio that tracks the decrease in the very fine-grained magnetite component. Also parallel to these is a trend toward heavier δ13C values in foraminiferal tests. The increase in organic carbon transport to the sediments led to chemical dissolution of the finest grain-size fraction of magnetite in the Oligocene sediments, hence a reduction in the superparamagnetic component and the change in the rock magnetic ratio. In this way, rock magnetics can be sensitive indicators of environmental changes, such as fluctuations in organic carbon transport, which may leave little other trace in the sedimentary record.

[1] Significant uncertainties persist in the reconstruction of past sea surface temperatures in the eastern equatorial Pacific, especially regarding the amplitude of the glacial cooling and the details of the post-glacial warming. Here we present the first regional calibration of alkenone unsaturation in surface sediments versus mean annual sea surface temperatures (maSST). Based on 81 new and 48 previously published data points, it is shown that open ocean samples conform to established global regressions of U37K′ versus maSST and that there is no systematic bias from seasonality in the production or export of alkenones, or from surface ocean nutrient concentrations or salinity. The flattening of the regression at the highest maSSTs is found to be statistically insignificant. For the near-coastal Peru upwelling zone between 11–15°S and 76–79°W, however, we corroborate earlier observations that U37K′ SST estimates significantly over-estimate maSSTs at many sites. We posit that this is caused either by uncertainties in the determination of maSSTs in this highly dynamic environment, or by biasing of the alkenone paleothermometer toward El Nino events as postulated by Rein et al. (2005).

[1] Orbital tuning, the process of fitting sedimentary cycles to orbital periodicities, can estimate with high resolution the timing and duration of key events in the geological record. We formulate here orbital tuning as the inverse problem of finding the variation in sedimentation rate that matches sediment cycles with orbital periodicities. Instead of obtaining a single best estimate, we apply a Bayesian formulation and define a probability distribution of sedimentation rate variations that result in powerful orbital periodicities. By sampling this distribution with a Monte Carlo method, we quantify the uncertainty in the inferred sedimentation rates due to uncertainties in the tuning periods and to components of the sedimentary signal that are unrelated to orbital cycles. The method is applied to the chronology of a 30 m interval in the Cismon APTICORE borehole (Southern Alps, Italy) that includes the early Aptian oceanic anoxic event 1a (Selli Level), estimated to last 1.11 ± 0.11 Ma (95% interval). The δ13C record shows a sudden negative shift of about −1‰ at the base of the Selli Level (22–47 ka) followed by a recovery to preshift values in ∼240 ka, consistent with a scenario where light carbon is quickly added and then flushed out of the ocean-atmosphere system.

[1] The Bering Sea gateway between the Pacific and Arctic oceans impacts global climate when glacial-interglacial shifts in shore line position and ice coverage change regional albedo. Previous work has shown that during the last glacial termination and into the Holocene, sea level rises and sea ice coverage diminishes from perennial to absent. Yet, existing work has not quantified sea ice duration or sea surface temperatures (SST) during this transition. Here we combine diatom assemblages with the first alkenone record from the Bering Sea to provide a semiquantitative record of sea ice duration, SST, and productivity change since the Last Glacial Maximum (LGM). During the LGM, diatom assemblages indicate that sea ice covered the southeastern Bering Sea perennially. At 15.1 cal ka B.P., the diatom assemblage shifts to one more characteristic of seasonal sea ice and alkenones occur in the sediments in low concentrations. Deglaciation is characterized by laminated intervals with highly productive and diverse diatom assemblages and inferred high coccolithophorid production. At 11.3 cal ka B.P. the diatom assemblage shifts from one dominated by sea ice species to one dominated by a warmer water, North Pacific species. Simultaneously, the SST increases by 3°C and the southeastern Bering Sea becomes ice-free year-round. Productivity and temperature proxies are positively correlated with independently dated records from elsewhere in the Bering Sea, the Sea of Okhotsk, and the North Pacific, indicating that productivity and SST changes are coeval across the region.

Bedded pelagic limestones of early to mid Cretaceous age may record cyclic climate variability driven by changes in the Earth's orbit (“Milankovitch cycles”). If properly calibrated, the lithologic cycles could provide sedimentary chronometry at far shorter time scales than biostratigraphy and magnetic stratigraphy. The lower Cretaceous Maiolica Formation of northern and central Italy displays rythmic bedding in sections whose magnetic reversal sequence is clearly correlated to the upper M-series marine anomalies. The magnetic reversals divide measured sections into 0.5–2 Myr increments in which sedimentation rate (assuming continous deposition) can be measured to 15–25% accuracy. Paleomagnetically estimated sedimentation rates over a number of polarity zones establish, with one exception, a narrow range in periods of the carbonate cycles. Carbonate couplets have an estimated mean period of 23.5 kyr, and prominent modulations in bedding thickness (“bundling”) have a mean period of 117 kyr (Kent and Gradstein 1985 time scale). The close match of sedimentary periodicities to orbital repeat times implies that the carbonate cycles reflect a combination of professional and eccentricity climatic forcing. Comparison of measured sections from different locations shows that bed-to-bed correlations are possible regionally, cosistent with the Milankovitch hypothesis. Episodes of Barremian “black shale” deposition occur in the troughs of the short eccentricity cycle, as roughly one half of the carbonate-marl couplet, or about 10 kyr duration. It is possible to estimate both the timing (relative to the top of reversed chron M-0) and duration of a period of unusual organic carbon-rich deposition (“Selli Horizon”) in the early part of the Cretaceous Long Normal Magnetic Polarity Interval by tracing the sedimentary cycles up from the Barremian strata.

[1] We present new alkenone-based sea surface temperature (SST) estimates from the eastern equatorial Pacific (EEP) for the last 30 kyr. By combining these new results with recently published records from the region, we reconstruct the spatial pattern of changes in SST during the Last Glacial Maximum (LGM). Alkenone-based SST estimates show a greater glacial cooling in the upwelling environment of the cold tongue than in sites located further north in the equatorial front and eastern Pacific Warm Pool. This result agrees with the paradigm of stronger glacial winds, increased upwelling, steeper zonal thermocline tilt, and stronger advection of cold water in the Peru Current. Furthermore, we investigate possible changes in glacial surface hydrography by using the alkenone-based SST reconstructions to correct planktonic foraminifera δ18O for the temperature effect. After additional correction for the global ice volume effect, the residual changes in seawater δ18O show a clear latitudinal pattern that would be consistent with a southward shift of the Intertropical Convergence Zone. We thus suggest that changes in sea surface salinities could explain contrasting SST reconstructions based on planktonic foraminifera δ18O, which implied a weakening of the cold tongue. The controversial LGM dynamics of the EEP reconstructed by different proxies, i.e., a weakening or a strengthening of the cold tongue, highlight the necessity to better assess the influence of various biases on these proxies.

[1] We provide the first continuous, orbital-resolution sea surface temperature (SST) record from the high-latitude North Atlantic, a region critical to understanding the origin of the Plio-Pleistocene ice ages and proximal to regions that became frequently glaciated after ∼2.7 Ma. We analyzed sediments from Ocean Drilling Program Site 982 over the last 4 Ma for their alkenone unsaturation index and compared this surface water signal to a benthic δ18O record obtained from the same section. We find that while ocean surface temperatures were significantly warmer (∼6°C) than modern temperatures during the early Pliocene, they were also as variable as those during the late Pleistocene, a surprising result in light of the subdued variance of oxygen isotopic time series during the interval of 3–5 Ma. We propose two possible explanations for the high orbital-scale SST variability observed: either that a strong, high-latitude feedback mechanism not involving large continental ice sheets alternately cooled and warmed a broad region of the northern high latitudes or that by virtue of its location near the northern margin of the North Atlantic Drift, the site was unusually sensitive to obliquity-driven climate shifts. On supraorbital time scales, a strong, sustained cooling of North Atlantic SSTs (∼4.5°C) occurred from 3.5 to 2.5 Ma and was followed by an interval of more modest cooling (an additional 1.5°C) from 2.5 Ma to the present. Evolutionary orbital-scale phase relationships between North Atlantic SST and benthic δ18O show that SST began to lead δ18O significantly coincident with the onset of strong cooling at Site 982 (∼3.5 Ma). We speculate that these changes were related to the growth and subsequent persistence of a Greenland ice sheet of approximately modern size through interglacial states.

[1] Although different paleoenvironmental time series resolve past climatic change at different time scales, nearly all share one characteristic: they are nonstationary over the length of the record sampled. We describe a recursive dynamic programming change point algorithm that is well suited to identify shifts in the Earth system’s variability, as it represents a nonstationary time series as a series of regimes, each of which is homogeneous. The algorithm fits the data by minimizing squared errors not only over the parameters of the models for each subsequence but also over an arbitrary number of boundary points without restrictions on the lengths of regimes. The versatility of the algorithm is illustrated by an application to 5 Ma of Plio-Pleisotcene d 18 O variations. We seek to identify either the single dominant ‘‘Milankovitch’’ frequency or linear combinations of frequencies and consistently identify changes � 780 ka and � 2.7 Ma, among others, in each analysis done. Our applications also provide support to the recent hypothesis that obliquity-based Milankovitch terms can account for the circa 100 ka cycle that empirically dominates the most recent 1 million years.

Both absolute and elapsed time can be measured to very high precision in sediments that contain climatic signals paced by regular changes in the Earth's orbit, because the orbital changes occur at well-known frequencies. The process of matching climatic signals recorded in sediments to a time template of orbital pacing is referred to as orbital tuning. Tuning involves techniques that range from simple pattern recognition to sophisticated methods of spectral (frequency) analysis. Although orbital tuning derives from statistical analyses of late Pleistocene Ice Age cycles, it has been successfully applied to sedimentary sequences over much of the 200 My.

We report well-dated Late Cretaceous and Early Tertiary precessional climatic cycles, recorded by rhythmic carbonate maxima and minima in South Atlantic deep sea sites. Spectral analyses of digitized sediment color, a suitable carbonate proxy, show prominent regularities in the spacing marl-carbonate beds. Magnetostratigraphic dating over a number of magnetic chrons constrains the duration of the cycles, which can be detected over at least 20 Myr of sedimentation at 7 coring locations. Their mean absolute period of23.5 ± 4.4kyr agrees closely with the predicted late Cretaceous precessional period of 20.8 kyr. Because they can be matched to a physical forcing mechanism with a known repeat time, the cycles offer a new high-resolution tool to measure rates of climate change before and after the Cretaceous-Tertiary (K/T) boundary. From counts of carbonate cycles, we derive the position of the K/T boundary within C29R at 350 kyr after the base of the reversal. The constancy of cycle thickness (linearly related to sedimentation rate) and amplitude up to the “boundary clay” does not give evidence for climate instability preceding the boundary. Orbital chronometry records a step-function decrease in sediment accumulation rate at the Cretaceous-Tertiary boundary that is consistent with a geologically instantaneous event.

[1] A composite section, which reconstructs a continuous stratigraphic record from cores of multiple nearby holes, and its associated composite depth scale are important tools for analyzing sediment recovered from a drilling site. However, the standard technique for creating composite depth scales on drilling cruises does not correct for depth distortion within each core. Additionally, the splicing technique used to create composite sections often results in a 10–15% offset between composite depths and measured drill depths. We present a new automated compositing technique that better aligns stratigraphy across holes, corrects depth offsets, and could be performed aboard ship. By analyzing 618 cores from seven Ocean Drilling Program (ODP) sites, we estimate that ∼80% of the depth offset in traditional composite depth scales results from core extension during drilling and extraction. Average rates of extension are 12.4 ± 1.5% for calcareous and siliceous cores from ODP Leg 138 and 8.1 ± 1.1% for calcareous and clay-rich cores from ODP Leg 154. Also, average extension decreases as a function of depth in the sediment column, suggesting that elastic rebound is not the dominant extension mechanism.

[1] The nature of the connection between high- and low-latitude climates during the early Pleistocene “41 kyr world” has important implications for our understanding of the feedbacks involved in translating insolation changes into global climate states. Here we focus on the tropical marine record, presenting alkenone-derived sea surface temperature (SST) and productivity records from the eastern equatorial Atlantic, eastern equatorial Pacific, the Arabian Sea, and the South China Sea for a time interval covering the heart of the 41 kyr world (1.2–2.0 Ma). All four SST records are dominated by variance in the obliquity band, suggesting that high-latitude dynamics and low-latitude climate were tightly coupled in the 41 kyr world, despite smaller ice volume variability during this interval as compared to the late Pleistocene. At the 41 kyr period, SST varied coherently and nearly synchronously between the four study regions, suggesting a tropic-wide feedback to high-latitude processes. Productivity variations at our equatorial Atlantic and Pacific sites were also coherent in the obliquity band, implying tropical trade wind variability at this frequency during the early Pleistocene. In contrast, we observe heterogeneous SST and productivity responses in the precession band between each of the tropical locations. Local atmospheric circulation patterns, rather than a globally coordinated response to precessional insolation forcing, apparently determined SSTs and productivity in the tropics at precessional frequencies during the early Pleistocene.

[1] Marine sedimentary records of the last glacial from tropical monsoon latitudes indicate climate fluctuations comparable to rapid changes in δ18O recorded in Greenland. Synchronizing two high-resolution sedimentary records from the Oman and Pakistan margins, we resolve millennial-scale reversals in sea surface temperature (SST) gradient (ΔSST) across the Arabian Sea which directly correspond with the majority of Dansgaard-Oeschger (D-O) events for the last 65 kyr. The relative amplitude of individual monsoon and D-O events appears comparable, suggesting coupled and at least hemispheric forcing. To explore this quasi-cyclic forcing, we compare alkenone-based geologic data with modern satellite-derived ΔSST estimates between sites. Interstadial conditions fall within the range of monsoons during the Holocene, but stadial conditions have no analogs. Following published associations between Eurasian Winter Snow Cover (EWSC) and monsoon rainfall, and El Nino/Southern Oscillation (ENSO) events and anomalous EWSC, we find a good correlation between the Southern Oscillation Index (SOI) and Arabian Sea ΔSST throughout the modern data set. An apparent 11-year cyclicity in the SOI reveals an association between the monsoon, SOI, and solar output variability. The SOI primarily tracks solar total irradiance, but the SOI monsoon linkage becomes nonlinear during excursions of the SOI associated with El Nino Events. Strong El Ninos coincide precisely with minimum solar irradiance during the solar cycle, which we attribute to threshold behavior in tropical Pacific SSTs and associated trade wind strength. We propose that both short-term (interannual-decadal) and long-term (centennial-millennial) changes in solar output are consistent with records of ENSO variability, monsoon intensity, and D-O event timing.

The mid-Cretaceous (Albian) deep-water sediments (coccolith–globigerinacean marls) of the Umbria–Marche Apennines show complex rhythmic bedding. We integrated earlier work with a time-series study of a digitized and image-processed photographic log of the Piobbico core. A drab facies is viewed as recording normal stratified conditions, and a red facies as the product of downwelling warm saline (halothermal) waters. Both are pervaded by orbital (Milankovitch) rhythms. These reflect fluctuations in the composition and abundance of the calcareous plankton in the upper waters. The drab facies is overprinted by redox oscillations on the bottom, including episodic precessional anaerobic pulses (PAPs). Contrasts between the individual beds representing the alternate phases of the precessional rhythm rose and fell with orbital eccentricity, in the classical pattern of Berger’s climatic precession or precession index curve, varyingly complicated by the obliquity rhythm. We conclude that greenhouse oceans in general, and perhaps this area in particular, were very sensitive to orbital forcing. Our count of 29 406-ky eccentricity cycles yields an Albian duration of 11.8 ± 0.4 My. Cyclostratigraphy: Approaches and Case Histories SEPM Special Publication No. 81, Copyright © 2004 SEPM (Society for Sedimentary Geology), ISBN 1-56576-108-1, p. 57–8

Holocene and latest Pleistocene oceanographic conditions and the coastal climate of northern California have varied greatly, based upon high-resolution studies (ca. every 100 years) of diatoms, alkenones, pollen, CaCO 3 %,and total organic carbon at Ocean Drilling Program (ODP) Site 1019 (41.682°N, 124.930°W, 980 m water depth). Marine climate proxies (alkenone sea surface temperatures [SSTs] and CaCO 3 %) behaved remarkably like the Greenland Ice Sheet Project (GISP)-2 oxygen isotope record during the Balling-Allerod, Younger Dryas (YD), and early part of the Holocene. During the YD, alkenone SSTs decreased by >3°C below mean Bolling-Allerod and Holocene SSTs. The early Holocene (ca. 11.6 to 8.2 ka) was a time of generally warm conditions and moderate CaCO 3 content (generally >4%). The middle part of the Holocene (ca. 8.2 to 3.2 ka) was marked by alkenone SSTs that were consistently 1-2°C cooler than either the earlier or later parts of the Holocene, by greatly reduced numbers of the gyre-diatom Pseudoeunotia doliolus (

The Cretaceous/Tertiary (K/T) impact radically changed the long-term state of the open-marine ecosystem. Comparison of range data from multiple sites indicates that more than 90% of planktic foraminiferal species went extinct at or shortly after the time of the K/T impact. Nearly complete replacement of planktic foraminiferal faunas began at the impact horizon. This turnover resulted in the disappearance of onshore-offshore provincialization of lowand mid-latitude planktic foraminiferal faunas. Drastic decreases in calcareous nannofossil production and organic export from surface to deep waters coincided with this destruction of faunal provincialization. Precessional chronometry of South Atlantic sites indicates that the initiation of faunal replacement and the decrease in nannofossil production occurred in an unresolvably short interval of time. Recovery of planktic foraminiferal assemblages was very dynamic. A rapid burst of first occurrences closely followed the impact. Despite the rapidity of this evolutionary radiation, long-term assemblage stabilization and recovery of organic export did not occur for at least a few hundred k.y. after the impact. Calcareous nannoplankton production remained below preimpact levels for two million years or more. There is some evidence that organic export to deep waters remained below preimpact levels for a similar interval of time. The coincidence of the K/T impact with planktic foraminiferal mass extinction, faunal turnover, and strong decreases in organic export and nannofossil production is very compatible with an impact model of mass extinction. In contrast, the proximate causes of the geologically long delay in postimpact recovery of organic export and nannofossil production remain unresolved. This presents a major challenge to our understanding of the ecologic and oceanographic consequences of large impacts and mass extinctions. D’Hondt, S., Herbert, T. D., King, J., and Gibson, C., 1996, Planktic foraminifera, asteroids, and marine production: Death and recovery at the CretaceousTertiary boundary, in Ryder, G., Fastovsky, D., and Gartner, S., eds., The Cretaceous-Tertiary Event and Other Catastrophes in Earth History: Boulder, Colorado, Geological Society of America Special Paper 307.

An 8-m section of a mid-Cretaceous (Albian) deep-sea sediment core from Piobbico, Italy contains ∼2 cycles/m compositional oscillations. As postdepositional chemical migration in the core is weak, the relative amount of carbonate is a primary indicator of Cretaceous paleoclimate variability. We report results of spectral analysis of two series, CaCO3 wt % measurements taken at roughly 2 cm intervals and a more densely sampled photodensitometer record of light-dark variation. The uncertainty in the age-depth relation (i.e., unknown fluctuations in sedimentation rate) hampers a detailed assessment of the suggested eccentricity correlation. We fit for long-term (time scale greater than 75 kyr) fluctuations in sedimentation rate by tracking the local frequency modulation of the dominant harmonic in the data series, assuming the true input signal to be monochromatic. We estimate uncertainty in the observed local frequency by means of a statistical jackknife. We fit a smoothed cubic spline to the set of discrete frequency modulation estimates, specifying a χ2 = N misfit criterion, where N is the number of frequency estimates. From this analysis we infer fluctuations of a factor of two in the sedimentation rate, which occur predominately on a time scale of 400 kyr. We generate “tuned” series using several candidate age-depth relations from the carbonate and densitometer series. The tuned carbonate series returns greater correlation with the modern Milankovitch periodicities. The 2 cycles/m energy resolves principally into two harmonic components whose frequency ratio corresponds (to an observational accuracy of 1–2%) to that of the 97.0- and 127.6-kyr eccentricity oscillations. At frequencies lower than 2 cycles/m, we find harmonic oscillations whose phase correlates with the “beating” envelope of the 2 cycles/m energy in a manner consistent with the modern Milankovitch insolation. We also observe a sinusoidal oscillation of lower amplitude with period (scaled by those of the presumed eccentricity oscillations) of 39.2 ± 1.1 kyr, which agrees with the period of the modern obliquity cycle after correction for the effects of tidal friction. Oscillations corresponding to Milankovitch precessional frequencies are not established, owing possibly to low signal level and unknown short-term deposition fluctuations. The tuned densitometer series recovers few new periodic features, although its inferred age-depth relation correlates well with that of the carbonate series. We calibrate the depth variable downcore to the orbital cycles to estimate r=5.0 ± 0.15 m/m.y. as the effective sedimentation rate.

Albian pelagic sediments from the Umbrian Apennines exhibit rhythmic bedding in outcrop. High temporal resolution (∼ 4-kyr spacing) sampling of carbonate, Si, and Al along a 1.6-m.y. interval of core demonstrates that changes in accumulation rate of calcareous and siliceous microorganisms were controlled by insolation cycles. Optical densitometry logging correlates variations of geochemical parameters to episodes of deep-sea anoxia, indicated by numerous black, laminated zones (“black shales”). Harmonic analysis of data shows oscillations at three frequencies, which correspond closely in estimated duration and ratio to the modern eccentricity and precessional orbital cycles. As in Pleistocene marine sequences, the eccentricity (approximately 100 kyr) term dominates the sedimentary variance in the Umbrian cycles. Anoxic episodes occur in phase with a limestone-marl repetition and reflect minima of the precessional (21 kyr) cycle. The covariance of carbonate and silica, with estimates of foraminiferal abundances, suggest that carbonate cycles are the result of changes in surface productivity of calcareous organisms rather than an effect of dissolution or diagenesis. Recognition of orbital cyclicity provides a tool for making quantitative estimates of these changes. Productivity during anoxic events was uniformly low (0.4– 0.5 g/cm2/kyr carbonate, 0– .05 g/cm2/kyr silica), but it rose to high levels during oxygenated periods (1– 2.5 g/cm2/kyr carbonate, 0.1– 0.3 g/cm2/kyr silica). Vertical overturning and organic carbon flux decreased during anoxic episodes, but decreased oxygenation of the deep water led to enhanced preservation of the organic matter delivered. The degree of paleoceanographic variability indicated by this study requires that accepted notions of climate stability and oceanic quiescence during nonglacial times be reassessed.

The Earth's orbital variations, reflected in Pleistocene ice volume, were also recorded in non-glacial times. Carbonate production in pelagic mid-Cretaceous sediments, quantified by calcium carbonate and optical densitometry time series, reflects the orbital eccentricity and precessional cycles. Minimal eccentricity brought deep-sea anoxia. Most of the sedimentary variability of this 100-Myr-old sequence lies within the Milankovitch (orbital) frequency band.

The Piobbico core of Aptian-Albian pelagic rhythmites in Italy has been used to explore ways of extracting quantitative time-series of geological, chemical, physical and biological parameters from stratigraphic sequences. Recognition of the precession, obliquity, and eccentricity cycles (the Option frequencies) has permitted time-resolution to the 104-year level. Individually, these time-series curves furnish a potential basis for precise correlations. Collectively, they track the evolution of the depositional system in detail, including its response to orbital forcing. Orbital cycles primarily influenced oceanic fertility and carbonate productivity by foraminifera and coccoliths, as well as the aeration of bottom waters (redox cycle). Three short magnetic reversals occur w thin the Ticinella praeticinensis zone. Frequency spectra of variations in magnetic intensity, inclination and declination yield Milankovitch frequencies which are only partly explained as responses to mineralogy. A computer simulation used Berger's astronomically calculate precession index to drive 1) carbonate flux and 2) depth of bioturbation. This achieved a stratigraphy remarkably similar to that observed. Each step in the computer simulation transfers spectral power from precession to eccentricity. Work on the Piobbico core continues, while Project ALBICORE aims to explore the patterns of rhythmicity in the praeticinensis zone of other areas. End_Page 1164