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Temporal productivity contents (including contents of brassicasterol, dinosterol, akenones). Units are in ng/g. Total Organic Carbon (TOC) contents were expressed in %. Temperature was expressed in℃. Nutrient contents were expressed in mmol/m3.

Variations in East Asian summer monsoon (EASM) precipitation impact agriculture, water resources, electricity generation, and economic development in the densely populated region of central eastern China. However, uncertainties remain in the history and driving mechanisms of precipitation changes in this region over the past two millennia. We present an 1800 yr multiproxy reconstruction of monsoon precipitation based on hydroclimate variations inferred from Lake Nvshan sediments in the Jiang-Huai region of central eastern China. We find that rainfall in the Jiang-Huai region was higher during the Little Ice Age (LIA) than during the Medieval Warm Period (MWP), and that these changes were modulated by the mean state of the tropical Pacific Ocean on centennial time scales. We infer that an El Niño–like (La Niña–like) mean state caused a strengthening and westward displacement (weakening and eastward displacement) of the Western Pacific Subtropical High (WPSH) and a weakening (strengthening) of the EASM, causing more (less) rainfall in the Jiang-Huai region during the LIA (MWP). These hydroclimate changes are likely to have been a response to changes in effective radiative forcing.

Abatract More than 80% of marine organic carbon (OC) burial occurs in sediments of marginal seas. Sedimentary OC 14C ages up to several millennia older than co-deposited coastal and pelagic sediments have been well documented but the cause of this phenomenon remains uncertain. We measured 14C and 13C contents of OC, along with the sedimentary content of terrestrial and marine lipid biomarkers, in sediment cores from the East China Sea to evaluate and quantify processes controlling OC ages over the last 14.3 kyr. We find that 14C ages of OC were persistently older than co-deposited sediments by 1930–5530 yr. Temporal variations of the calculated apparent initial radiocarbon ages of total OC (TOC-AIR) mirrored sea level changes, with higher values (4570 ± 1250 yr) during the transgression (14.1–7.8 kyr BP) and lower values (3170 ± 670 yr) during the mid-late Holocene (7.8–0 kyr BP), suggesting that transgression-induced coastline retreat reduced the transport of pre-aged terrestrial OC to the marginal sea. However, bulk OC 14C ages were consistently older than those expected from ternary mixing of Changjiang (Yangtze River), Huanghe (Yellow River) and marine sources based on δ13C and Δ14C end-members. We therefore propose that hydrodynamic processes during sediment transport and the addition of pre-aged OC from land and submerged coast were the main factors contributing to these old 14C ages of OC and their temporal variations. During the transgression, higher TOC-AIR values were observed during times of lower sea-level, which suggests that erosion of coastal deposits during transgression might have contributed old OC. Since sea level stabilized 7.8 kyr BP hydrodynamic processes were the primary cause of high 14C ages of OC. The significant role for hydrodynamic aging processes during the mid-late Holocene is hypothesized to result from longer transport distances between river mouths and sediment depocenters which can accommodate additional deposition-resuspension loops.

Hydroclimate on ‘Uvea (Wallis et Futuna) is controlled by rainfall associated with the South Pacific Convergence Zone (SPCZ), the southern hemisphere's largest precipitation feature. To extend the short observational precipitation record, the hydrogen isotopic composition of the algal lipid biomarker dinosterol (δ2Hdinosterol) was measured in sediment cores from two volcanic crater lakes on ‘Uvea. The modern lakes differ morphologically and chemically but both contain freshwater within the photic zone, support phytoplankton communities inclusive of dinosterol-producing dinoflagellates, and experience identical climate conditions. δ2Hdinosterol values track lake water isotope ratios, ultimately controlled in the tropics by precipitation amount and evaporative enrichment. However, in 88-m-deep Lac Lalolalo a steadily decreasing trend in sedimentary δ2Hdinosterol values from −227‰ around year 988 CE to modern values as low as −303‰, suggests this lake's evolution from an active volcanic setting to the present system strongly influenced δ2Hdinosterol values. Although current hydrology and water isotope systematics may now reflect precipitation and evaporation in this lake, the interaction between theseprocesses and large changes in basin morphology, geochemistry, and hydrology obstruct the recovery of a climate signal from Lac Lalolalo's sedimentary δ2Hdinosterol records. This work emphasizes the importance of site replication and the use of complementary climate reconstruction tools, especially when using molecular proxiesthat may be sensitive to more than one environmental parameter. Contrary to its neighbor, duplicate δ2Hdinosterol records from 23-m-deep Lac Lanutavake varied between −277‰ and −297‰ and indicate slightly drier conditions during the time-period known as the Medieval Climate Anomaly (MCA, 950–1250 CE). The δ2Hdinosterol signal in Lac Lanutavake was muted compared to published records from ‘Upolu (Samoa) and Efate (Vanuatu) indicating that ‘Uvea's location is not as sensitive to precipitation variability at sites farther from the SPCZ centralaxis. Lithogenic runoff proxies combined with δ2Hdinosterol support the interpretation of a relatively dry MCA on ‘Uvea, ‘Upolu, and Efate, potentially due to less intense precipitation, a contracted, or a more zonally oriented SPCZ.

Paired hydrogen and carbon isotope ratios (2H/1H and 13C/12C) of mangrove lipids can be used to quantitatively reconstruct past salinity and 2H/1H ratios of environmental water, and in some cases precipitation rate. This approach is based on the observation that net 2H- and 13C-fractionation increases and decreases, respectively, with the salinity of environmental water. In order to better understand the mechanisms underlying these empirical observations and ultimately improve estimates of paleoprecipitation from the paired H and C isotope approach, we analyzed the isotopic composition of fatty acids from five species of mangroves cultivated in salinity treatments of 5–30 ppt (g/kg) for 3.5 years in a greenhouse. Decreased net 13C-fractionation with salinity in three mangrove species was attributed to increased water use efficiency and thus a 13C-enriched internal CO2 pool. Net 2H-fractionation decreased with salinity in three mangrove species, opposite to previous observations of mangroves growing along salinity gradients in lakes and estuaries. The difference between uncultivated and greenhouse-cultivated mangroves may result from variability of 2H/1H of environmental water in natural environments. In addition, decreased net 2H-fractionation with salinity could be due to temporal variability in 2H/1H of leaf water and timing of lipid production, and the use of stored carbohydrates in seeds. Due to the sensitivity of the salinity and 13C-fractionation relationship for calculating both salinity and water isotopes, optimization of mangrove lipid H and C isotopes as a paleohydrologic tracer may be best achieved through laboratory-based calibrations of the relationship between 13C-fractionation and salinity.

The ocean’s biological pump strongly influences atmospheric CO2 and regulates Earth’s climate. Determining the contribution of different phytoplankton groups to the biological pump over geological timescales remains an important, yet elusive, goal in biogeochemistry and organic geochemistry. Towards that end, source-specific lipid biomarkers can be used, but this approach requires the quantification of the biomarker-to-carbon ratio in different phytoplankton species, and under differing environmental conditions. We investigated responses of brassicasterol, dinosterol and C37 alkenones to three temperatures (15, 20 and 25 °C) and three N:P supply ratios (10:1, 24:1 and 63:1 mol mol−1) in three diatoms, three dinoflagellates and one coccolithophore, in laboratory experiments. Brassicasterol was produced by one diatom species, three dinoflagellates and the coccolithophore Emiliania huxleyi, while dinosterol and C37 alkenones were produced by dinoflagellates and E. huxleyi, respectively. Overall, carbon-normalized contents of lipid biomarkers varied by about a factor of three over the wide ranges of temperature and N:P supply ratios, in all species. Within the factor of three, brassicasterol was highest under the balanced N:P condition in diatoms, but under N and P deficiency in dinoflagellates. Brassicasterol in E. huxleyi was highest at lower temperatures. Dinosterol in dinoflagellates and C37 alkenones in E. huxleyi varied with temperature and N:P supply ratios, but not systematically. Compared to those in our experiments, smaller ranges are expected of N:P ratios and temperature and hence carbon-normalized biomarker contents at individual locations over time. Thus, our results imply that lipid biomarkers can be used to estimate taxon-specific carbon fluxes through time.

The hydrogen isotope ratio (2H/1H) of microalgal lipids has become a useful tool for reconstructing hydrologic conditions in the geologic past from marine and lacustrine sediments. Culture studies have shown that, in addition to the 2H/1H ratio of growth water, parameters such as salinity, growth rate, and irradiance, can have a large effect on lipid 2H/1H ratios. But aside from salinity, these effects have not been well studied in the field. In this work, hydrogen isotope fractionation relative to growth water was measured in four algal lipids - C37:3 and C37:2 methyl alkenones, (3β,4α,5α,22E)-4,23-Dimethylergost-22-en-3-ol (dinosterol), and 24-methylcholesta-5,22E-dien-3β-ol (brassicasterol) – in the upper water column of two North Pacific Ocean locations. By comparing lipid 2H/1H variations as a function of depth at tropical Station ALOHA (22°45′N, 158°00′W) and mid-latitude Gyre (41°30′N, 132°00′W in the Transition Zone) it was possible to evaluate the H isotope effects of nutrient limitation and irradiance on natural phytoplankton populations by comparing measured fractionation factors (αlipid) with those derived from laboratory-based empirical relationships. Significant distinctions were observed between the inferred causes of H isotope fractionation in different algal lipids at the two locations. At Station Aloha, 2H/1H fractionation in coccolithophorid-derived alkenones was primarily caused by nutrient-limited growth, while at Gyre it was caused by light limitation. Distinctions were less clear for the two sterols. 2H/1H fractionation in dinosterol was possibly influenced by nutrient-limited growth or light limitation at greater depths at both stations, while 2H/1H fractionation in brassicasterol matched values expected for nutrient-limited growth at both locations. Although systematic trends with depth in αlipid on the order of 10–30‰ were observed for each lipid, inter-day variability was some 3–7‰ depending on the lipid and site, and the magnitude of αlipid changes observed in the field were much less than those expected based on culture studies. This implies that the effects of nutrient and light limitation on natural phytoplankton populations in the ocean may not have a large influence on hydroclimate reconstructions based on 2H/1H ratios of microalgal lipids in marine sediments.

The South Pacific Convergence Zone (SPCZ) is the largest precipitation feature in the Southern Hemisphere supplying freshwater to 11 million people. Despite its significance, little is known about the location and intensity of SPCZ precipitation prior to instrumental records, hindering attempts to predict precipitation changes in a warming world. Here we use sedimentary molecular fossils to establish a tool for extending the historical record of precipitation. Freshwater lake sediments and water samples were collected from 30 lakes that span a 4.6 mm d−1 range in precipitation rates from the Global Precipitation Climatology Project (GPCP). 2Hlakewater values from 29 lakes ranged from –29 to +23‰ and were inversely correlated (r= −0.51, R2=0.26, p

This material is based upon work supported by the U.S. National Science Foundation under grant OCE-1241247 (J.P.S.). Additional funding was provided by the Australian Research Council under grant ARC DP120103202 (S.H. and G.C.). U.S. National Science Foundation grant OCE-1241255 to Michael Dawson and Michael Beman supported Jessica Blois during fieldwork in Palau.

Hydrogen isotope ratios of sedimentary leaf waxes (δ2HWax values) are increasingly used to reconstruct past hydroclimate. Here, we add δ2HWax values from 19 lakes and four swamps on 15 tropical Pacific islands to an updated global compilation of published data from surface sediments and soils. Globally, there is a strong positive linear correlation between δ2H values of mean annual precipitation (δ2HP values) and the leaf waxes n‐C29‐alkane (R2 = 0.74, n = 665) and n‐C28‐acid (R2 = 0.74, n = 242). Tropical Pacific δ2HWax values fall within the predicted range of values based on the global calibration, and the largest residuals from the global regression line are no greater than those observed elsewhere, despite large uncertainties in δ2HP values at some Pacific sites. However, tropical Pacific δ2HWax values in isolation are not correlated with estimated δ2HP values from isoscapes or from isotope‐enabled general circulation models. Palynological analyses from these same Pacific sediment samples suggest no systematic relationship between any particular type of pollen distribution and deviations from the global calibration line. Rather, the poor correlations observed in the tropical Pacific are likely a function of the small range of δ2HP values relative to the typical residuals around the global calibration line. Our results suggest that δ2HWax values are currently most suitable for use in detecting large changes in precipitation in the tropical Pacific and elsewhere, but that ample room for improving this threshold exits in both improved understanding of δ2H variability in plants, as well as in precipitation., Plain Language Summary: Past precipitation patterns are difficult to reconstruct, limiting our ability to understand Earth’s climate system. Geochemists reconstruct past precipitation by measuring the amount of heavy hydrogen naturally incorporated into the waxy coating of leaves, which is preserved in mud that accumulates in lakes, soils, and oceans. Heavy hydrogen in leaf waxes is strongly correlated with local precipitation, allowing us to learn about rainfall intensity, temperature, and cloud movement. However, no existing calibration studies include sites from the tropical Pacific, home to the most intense rainfall on the planet and populations that rely on rain for drinking water and farming. We measured heavy hydrogen in leaf waxes from tropical Pacific islands and show that although values are within the global calibration error, no precipitation relationship exists within the region. Plant type distributions do not explain the lack of correlation, which is best attributed to poorly constrained estimates of heavy hydrogen in local rain and the relatively small range of variability within the region. At present, heavy hydrogen from ancient leaf waxes can show large changes in past precipitation, but improved process‐level understanding is needed to use this tool to understand smaller changes in the tropical Pacific and elsewhere., Key Points: Leaf wax 2H/1H ratios are correlated with mean annual precipitation 2H/1H ratios globally, but not in the tropical Pacific. Deviations from the global relationship between precipitation leaf wax 2H/1H ratios cannot be predicted from palynological assemblages. Small range and large uncertainties in estimates of tropical Pacific precipitation 2H/1H ratios likely account for poor correlations., Swiss National Science Foundation, National Science Foundation (NSF) http://dx.doi.org/10.13039/100000001, Natural Environment Research Council (NERC) http://dx.doi.org/10.13039/501100000270, Department of Education and Training, Australian Research Council (ARC) http://dx.doi.org/10.13039/501100000923, http://10.0.15.89/ethz-b-000412154

Significance We combine indicators from lake sediments with archaeological records that identify an earlier and incremental arrival of humans in East Polynesia than indicated by current models. We use lake sediments to reconstruct a quantitative, multiproxy hydroclimate sequences from Vanuatu, Samoa, and the Southern Cook Islands and combine these with published data to show that the timing of human migration into East Polynesia coincided with a prolonged drought. We postulate this regional drought was a significant contributory factor in eastward exploration and subsequent colonization of the Southern Cook Islands and beyond. The return of wetter conditions in East Polynesia after c. AD 1150 supported subsequent colonization of other central islands and, eventually, migration into far eastern and South Polynesia., The timing of human colonization of East Polynesia, a vast area lying between Hawai‘i, Rapa Nui, and New Zealand, is much debated and the underlying causes of this great migration have been enigmatic. Our study generates evidence for human dispersal into eastern Polynesia from islands to the west from around AD 900 and contemporaneous paleoclimate data from the likely source region. Lake cores from Atiu, Southern Cook Islands (SCIs) register evidence of pig and/or human occupation on a virgin landscape at this time, followed by changes in lake carbon around AD 1000 and significant anthropogenic disturbance from c. AD 1100. The broader paleoclimate context of these early voyages of exploration are derived from the Atiu lake core and complemented by additional lake cores from Samoa (directly west) and Vanuatu (southwest) and published hydroclimate proxies from the Society Islands (northeast) and Kiribati (north). Algal lipid and leaf wax biomarkers allow for comparisons of changing hydroclimate conditions across the region before, during, and after human arrival in the SCIs. The evidence indicates a prolonged drought in the likely western source region for these colonists, lasting c. 200 to 400 y, contemporaneous with the phasing of human dispersal into the Pacific. We propose that drying climate, coupled with documented social pressures and societal developments, instigated initial eastward exploration, resulting in SCI landfall(s) and return voyaging, with colonization a century or two later. This incremental settlement process likely involved the accumulation of critical maritime knowledge over several generations.

Mangrove systems represent important long-term sinks for carbon since they have much higher carbon burial rates than terrestrial forests or typical coastal ecosystem. However, quantifying the sources of organic matter (OM) in estuarine and coastal sediments, where mangroves occur but are not the only source of OM, is challenging due to the variety of OM sources and diverse transport processes in these dynamic environments. The sources of OM in surface sediments of two mangrove-fringed estuaries in Hainan Province, China, were investigated using the mangrove specific biomarker taraxerol and other lipid biomarkers, as well as stable carbon isotopes. Mixing models based on the concentration of taraxerol, plant wax n-alkanes and δ13 COM indicate that terrestrial non-mangrove plant OM accounted for 52-72% of the OM in the two estuaries, aquatic OM from phytoplankton and/or seagrass accounted for 8-29%, and OM from mangroves comprised 16-26% of the total. Terrestrial plants contributed 16-20% more of the OM to sediments of Bamen Bay, which is on the wetter, eastern side of Hainan Island, than to Danzhou Bay, but aquatic OM (algae plus seagrass) fraction was 17% lower than that in Danzhou Bay sediments. In both estuaries, mangrove and aquatic OM fractions increased seaward while the terrestrial OM fraction decreased. Terrestrial fraction in BMB sediments is 12% higher in summer compared to autumn, which is offset by a comparable reduction in the mangrove OM fraction, as well as higher aquatic OM fractions in both estuaries. This may be caused by enhanced river discharge, more efficient mangrove leaf litter transport offshore, and/or higher aquatic productivity. The biomarker and carbon isotope approach used here can be applied to semi-quantitatively estimate spatial and temporal variations of the sources of organic carbon in tropical estuarine and coastal sediments, a major sink for carbon in the ocean.

The hydrogen isotopic composition (2H/1H, or δ2H) of palmitic acid (PA) was measured in surface sediments from the Laptev and Kara Seas in the Russian Arctic to evaluate its use as a paleohydrographic proxy. δ2HPA values in surface sediments varied by 118‰ over a 21 ppt range in mean annual surface salinity, and the two properties were highly correlated (R2 = 0.8, p

Acid reef-flux The uptake of anthropogenic carbon dioxide from the atmosphere is reducing the pH of the oceans. Ocean acidification means that calcium carbonate—the material with which coral reefs are built—will be more difficult for organisms to generate and will dissolve more quickly. Eyre et al. report that some reefs are already experiencing net sediment dissolution. Worryingly, the rates of loss will increase as ocean acidification intensifies. Science , this issue p. 908

Statements made at the recent UN Climate Conference 23rd session of the Conference of the Parties (COP23) clearly indicated that Pacific islands, countries, and territories (PICTs) are particularly susceptible to sociocultural, economic and environmental impacts of climate change. The Federated States of Micronesia (FSM) are one such group of islands where internal and external climate forcing has observable detrimental impacts on local public health, water quality, agriculture and resource management. Here, we present the outcomes of a collaboration between graduate students and a PICTs-focused non-profit organization to facilitate a climate-related knowledge network that addresses adaptation to climate-related vulnerabilities in Pohnpei, FSM. Through a series of workshops targeting K-8 science teachers, this network strengthens lines of communication between educators, resource managers, stakeholders and environmental leaders and provides a forum for ongoing information exchange to encourage adaptation to climate change in island communities. Additionally, teacher participation in the workshops resulted in a marked increase in community engagement in other local and regional educational venues. We propose that the knowledge network piloted here serves as an interdisciplinary model of a sustainable educational partnership that can be adapted for use in a multitude of PICT communities to improve preparedness and reduce susceptibility to climate-associated stressors, thereby providing an example of means to achieve key goals of the COP23. Keywords: Knowledge network, Climate change, Climate variability, Adaptive capacity, Learning exchange, Pacific islands

The effect of salinity on hydrogen isotope fractionation during the production of leaf wax n-alkanes was assessed for Laguncularia racemosa (white mangrove), Rhizophora mangle (red mangrove), and Avicennia germinans (black mangrove) along a 31 ppt (parts per thousand) salinity gradient in the Shark River estuary, Florida, USA. Significant variation in hydrogen isotope ratios was observed among these three Atlantic-East Pacific (AEP) species, with increasing leaf wax n-alkane 2H/1H fractionation with increasing salinity. Net 2H/1H fractionation for hentriacontane (n-C31) increased by 0.8, 1.4 and 1.8‰/ppt in R. mangle, A. germinans and L. racemosa, respectively. The observations are consistent with published δ2HnC31 data from 5 species of Indo-West Pacific (IWP) mangroves, which increased with salinity by 0.7–1.5‰/ppt. Although all measured species from both the AEP and IWP regions have more 2H/1H fractionation at high salinity, differences in slope and intercepts of these relationships are observed among genera. The differences may result from variation in the composition of compatible solutes, reliance on storage carbohydrates, and/or physiological response to salt. However, no statistically significant difference in the sensitivity of δ2HnC31 to salinity was observed in four Rhizophora species from both Indo-West Pacific and Americas-East Atlantic regions, which makes sedimentary Rhizophora lipids a promising target for paleohydroclimatic reconstruction.

Continuous cultures of the marine diatom Thalassiosira pseudonana were grown at irradiances between 6 and 47 µmol m −2 s −1 in order to evaluate the effect of light on hydrogen isotope fractionation in lipids. δ 2 H values increased with irradiance in phytol by 1.1‰ (µmol m −2 s −1 ) −1 and by 0.3‰ (µmol m −2 s −1 ) −1 in the C14:0 fatty acid, but decreased by 0.8‰ (µmol m −2 s −1 ) −1 in the sterol 24-methyl-cholesta-5,24(28)-dien-3β-ol (C 28 Δ 5,24(28) ). The anticorrelation between δ 2 H values in C 28 Δ 5,24(28) and irradiance is attributed to enhanced sterol precursor synthesis via the plastidic methylerythritol phosphate (MEP) pathway at high irradiance, relative to the cytosolic mevalonic acid (MVA) pathway, and the supposition that MEP precursors are 2 H-depleted compared to MVA precursors because they incorporate a greater proportion of hydrogen from photosynthetically produced NADPH. Increasing δ 2 H values of phytol and C14:0 with irradiance is attributed to a greater proportion of pyruvate, the last common precursor to both lipids, being sourced from glycolysis in the mitochondria and cytosol, where enhanced incorporation of metabolic NADPH and further hydrogen exchange with cell water can enrich pyruvate with 2 H relative to pyruvate from the chloroplast. Irrespective of the biosynthetic mechanisms responsible for the 2 H/ 1 H fractionation response to light, the high sensitivity of lipid δ 2 H values in T. pseudonana continuous cultures would result in −30‰ to +40‰ variations in δ 2 H over a 40 µmol m −2 s −1 range in sub-saturating irradiance if expressed in the environment, depending on the lipid.

Hydrogen isotope ratios (2H/1H or D/H) of sedimentary mangrove lipid biomarkers can be exploited as a quantitative proxy of past salinity and water isotopes. This approach is based on the observation that apparent 2H/1H fractionation between surface water and mangrove leaf lipids increases with surface water salinity. In order to better understand the mechanisms responsible for this empirical relationship, we analyzed the isotopic composition of surface water, xylem water, leaf water and leaf lipids from Bruguiera gymnorhiza mangroves growing around eleven marine lakes and a lagoon on the rock islands of Palau, spanning a salinity range of 5–32 parts per thousand (ppt). Net fractionation increased with increasing salinity for both nC31-alkane (0.7 ± 0.1‰ ppt−1) and for the pentacyclic triterpenoid lupeol (0.5 ± 0.2‰ ppt−1). These trends could not be attributed to changes in biosynthetic fractionation with salinity, but seem more likely to be due to increasing disequilibrium between xylem water and water vapor as salinity increases. In Palau’s humid climate, this most likely causes leaf water to become less 2H-enriched relative to surface water and to xylem water as salinity increases. This supposition is supported not only by measurements of leaf water 2H enrichment, but also by the correlation (R2 = 0.66) between leaf water isotopes and those of rain water, which are assumed to be in equilibrium with water vapor isotopes, and by the dependence of leaf water isotopes on water vapor isotopes in a Peclet-modified Craig-Gordon model. These results should inform the application of sedimentary mangrove lipid hydrogen isotope ratios to infer past hydroclimatic changes.

Enhanced ocean carbon storage during the Pleistocene ice ages lowered atmospheric CO2 concentrations by 80 to 100 ppm relative to interglacial levels. Leading hypotheses to explain this phenomenon invoke a greater efficiency of the ocean's biological pump, in which case carbon storage in the deep sea would have been accompanied by a corresponding reduction in dissolved oxygen. We exploit the sensitivity of organic matter preservation in marine sediments to bottom water oxygen concentration to constrain the level of dissolved oxygen in the deep central equatorial Pacific Ocean during the last glacial period (18,000–28,000 years BP) to have been within the range of 20–50 μmol/kg, much less than the modern value of ~168 μmol/kg. We further demonstrate that reduced oxygen levels characterized the water column below a depth of ~1,000 m. Converting the ice age oxygen level to an equivalent concentration of respiratory CO2, and extrapolating globally, we estimate that deep‐sea CO2 storage during the last ice age exceeded modern values by as much as 850 Pg C, sufficient to balance the loss of carbon from the atmosphere (~200 Pg C) and from the terrestrial biosphere (~300–600 Pg C). In addition, recognizing the enhanced preservation of organic matter in ice age sediments of the deep Pacific Ocean helps reconcile previously unexplained inconsistencies among different geochemical and micropaleontological proxy records used to assess past changes in biological productivity of the ocean.

Hydrogen isotope ratios (2H/1H or δ2H) were measured in lipid biomarkers from algal, plant and microbial sources in sediment cores from a lake and a peat bog on the small, sparsely-inhabited Washington Island (4°43′N, 160°25′W) to assess central equatorial Pacific hydroclimate conditions during the last millennium. High δ2H values in lipids from a variety of biological sources indicate that the driest period of the last millennium occurred ∼1450–1600 CE, during the first half of the Little Ice Age (LIA; 1450–1850 CE). An Intertropical Convergence Zone (ITCZ) located south of its modern position, less frequent and/or weaker El Nino events, and/or a La Nina-like mean state in the tropical Pacific are potential causes for this drying at Washington Island. From ∼1600 to 1650 CE, an abrupt transition to modern-like low δ2H values in phytoplankton, plant, and microbial lipids occurred, signaling the establishment of a high-rainfall regime similar to that observed today. This transition coincided with increased ENSO variability, an excess of central-Pacific relative to eastern-Pacific El Ninos, and a decline in the zonal SST gradient across the tropical Pacific, reflecting an El Nino-like mean state. The Medieval Warm Period (MWP; 900–1250 CE) was characterized by high δ2H values in lipids from phytoplankton (dinosterol, dinostanol), bacteria (hop-21-ene), and vascular plants (sitostanol), and by extension a drier climate relative to the modern lake. An increasing δ2H trend through the MWP in lipids from all sources implies drying as Northern Hemisphere temperatures declined from the early to the late MWP. This drying is hypothesized to have been driven by extensive volcanism in northern, southern, and tropical latitudes, all of which tend to cause zonal mean drying at the latitude of WI (5°N). Finally, the transition period between the LIA and MWP ∼1250–1450 CE was characterized by declining δ2H values of plant and microbial lipids in peat sediments, indicating a trend toward wetter conditions; in the absence of known internal or external climate forcings, this may have been a regional or local event. This study demonstrates that the application of compound-specific δ2H measurements of lipids from multiple biological sources and in multiple sedimentary archives from a single location can yield hydroclimate reconstructions with higher confidence than those based on single lipids. Such reconstructions are particularly important in the vast tropical Pacific, where few hydroclimate records exist.

The hydrogen isotopic ( 2 H/ 1 H) composition of lipids in microalgae is significantly depleted relative to extracellular water. While a variety of growth conditions influence the magnitude of 2 H-depletion, the effect of salinity is of particular interest due to the paleohydrological applications of lipid 2 H/ 1 H. In previous studies, lipid–water 2 H/ 1 H fractionation was shown to decrease as salinity increased, a response largely independent of lipid type, species, or setting. The mechanism responsible for this response remains uncertain, primarily because salinity is rarely isolated as the sole variable in laboratory cultivation experiments investigating hydrogen isotope systematics in microalgae. Here we report the lipid–water 2 H/ 1 H fractionation response to salinity in nutrient-replete continuous cultures of the centric diatom Thalassiosira pseudonana . In six cultures with the same growth rate at salinities between 14–40 ppt, lipid–water 2 H/ 1 H fractionation decreased linearly as salinity increased by 1.3‰/ppt in fatty acids (C 14:0 , C 16:0 , C 16:1 ) and by 1.0‰/ppt in the sterol 24-methyl-cholesta-5,24(28)-dien-3β-ol. A constant growth rate between cultures reveals that the fractionation response to salinity is independent of growth rate. Sensitivity tests using a simple hydrogen flux model indicated that at high salinity a greater proportion of metabolic NAD(P)H in lipids at the expense of photosynthetic NADPH can cause 2 H-enrichment. Additionally, increased exudate release or decreased hydrogen transport can enrich both lipids and cell–water in 2 H. The 1.0–1.3‰/ppt increase in lipid–water 2 H/ 1 H fractionation observed in T. pseudonana is within the 0.8–2‰/ppt range observed in field studies and culture studies, supporting the application of algal lipid 2 H/ 1 H as a paleosalinity proxy.

Salinity and temperature dictate the buoyancy of seawater, and by extension, ocean circulation and heat transport. Yet there remain few widely applicable proxies for salinity with the precision necessary to infer all but the largest hydrographic variations in the past. In the last decade the hydrogen isotope composition ( 2 H/ 1 H or δ 2 H) of microalgal lipids has been shown to increase systematically with salinity, providing a foundation for its use as a paleosalinity proxy. Culture and field studies have indicated a wide range of sensitivities for this response, ranging from about 0.6–3.3‰ ppt −1 depending on the lipid, location and/or culturing conditions. Lacking in these studies has been the controlled conditions necessary to isolate the response to salinity while keeping all other growth parameters constant. Here we show that the hydrogen isotope composition of lipids in the marine coccolithophorid Emiliania huxleyi grown in chemostats increased by 1.6 ± 0.3‰ ppt −1 ( p −1 ( p −1 . This sensitivity of δ 2 H alkenone to salinity is 45–71% of that previously reported for E. huxleyi , which can be attributed to the fact that previous experiments were performed with batch cultures in which growth rates and other parameters differed between salinity treatments. The underlying cause of this response to salinity remains unknown, but may result from changes in (1) the proportion of lipid hydrogen derived from NADPH versus water, (2) the proportion of lipid hydrogen derived from NADPH from Photosystem I versus the oxidative pentose phosphate pathway (and other metabolic sources), or (3) the δ 2 H value of intracellular water.

Modern seasonal and inter-annual precipitation variability in Palau is linked to both meridional movement of the Intertropical Convergence Zone (ITCZ) and changes in the Pacific Walker Circulation (PWC) associated with the El Nino–Southern Oscillation. Thus, Palau’s hydroclimate should be sensitive to mean shifts in the ITCZ and PWC on decadal to centennial time scales. Using compound-specific hydrogen isotope ratios (δ2H) of dinosterol in lake sediments, we generated a decadal-resolution proxy record of hydroclimatic variability in Palau spanning the past 800 yr. Results indicate a drying trend during the Little Ice Age in Palau, consistent with a southward displacement of the ITCZ. In addition to the secular drying trend, there are persistent large (∼20‰) multi-decadal to centennial oscillations in the δ2H record, the most recent of which indicates an abrupt shift to drier conditions in the mid-1970s that coincides with a decadal-scale negative shift in the Southern Oscillation Index.

The role of radiative forcings and climate feedbacks on global cooling over the last millennium is quantified in the CMIP5–PMIP3 transient climate model simulations. Changes in the global energy budget over the last millennium are decomposed into contributions from radiative forcings and climate feedbacks through the use of the approximate partial radiative perturbation method and radiative kernels. Global cooling occurs circa 1200–1850 CE in the multimodel ensemble mean with pronounced minima corresponding with volcanically active periods that are outside the range of natural variability. Analysis of the global energy budget during the last millennium indicates that Little Ice Age (LIA; 1600–1850 CE) cooling is largely driven by volcanic forcing (comprising an average of 65% of the total forcing among models), while contributions due to changes in land use (13%), greenhouse gas concentrations (12%), and insolation (10%) are substantially lower. The combination of these forcings directly contributes to 47% of the global cooling during the LIA, while the remainder of the cooling arises from the sum of the climate feedbacks. The dominant positive feedback is the water vapor feedback, which contributes 29% of the global cooling. Additional positive feedbacks include the surface albedo feedback (which contributes 7% of the global cooling and arises owing to high-latitude sea ice expansion and increased snow cover) and the lapse rate feedback (which contributes an additional 7% of the global cooling and arises owing to greater cooling near the surface than aloft in the middle and high latitudes).

Hydrogen isotopes in lipid biomarkers can trace past changes in the hydrologic cycle. Recent studies have revealed the potential of hydrogen isotopes in microalgal lipids for quantitatively reconstructing water δ 2H (δD) values and salinity. In this study we collected suspended particles along a salinity gradient from the Changjiang River Estuary (CRE), and measured δD values in fatty acids in these particles. The results indicated that δD values of water were correlated highly with salinity from the CRE, in agreement with the results from other estuaries. δD values in palmitic acid and stearic acid had a positive correlation with δD values of water from the CRE. Nevertheless, in the CRE, hydrogen isotope fractionation in fatty acids relative to water increased as salinity increased, opposite the trend in hydrogen isotope fractionation with salinity found in microalgal culture and field studies. We attribute the increase in hydrogen isotope fractionation as salinity increased to light availability, which was likely lower in the particle rich mixing zone at the end of the estuary, and potentially as well to multiple sources of fatty acids in the CRE.

Mangrove systems represent important long-term sinks for carbon since they have much higher carbon burial rates than terrestrial forests or typical coastal ecosystems. However, quantifying the sources of organic matter (OM) in estuarine and coastal sediments, where mangroves occur but are not the only source of OM, is challenging due to the variety of OM sources and diverse transport processes in these dynamic environments. The sources of OM in subtidal surface sediments of two mangrove-fringed estuaries in Hainan Province, China, were investigated using the mangrove-specific biomarker taraxerol and other lipid biomarkers, as well as stable carbon isotopes. Mixing models based on the concentration of taraxerol, plant wax n-alkanes and δ13COM indicate that terrestrial non-mangrove plant OM accounted for 40–57% of the OM in the two estuaries, phytoplankton OM accounted for 24–45%, and OM from mangroves comprised 15–19% of the total. Terrestrial plants contributed 10–21% more of the OM to subtidal sediments of Bamen Bay, which is on the wetter, eastern side of Hainan Island, than to Danzhou Bay, but the phytoplankton OM fraction was 16–24% lower than that in Danzhou Bay subtidal sediments. In both estuaries, mangrove and phytoplankton OM fractions increased seaward while the terrestrial OM fraction decreased. On a seasonal basis, lipid biomarker concentrations indicated a small change of OM sources. The biomarker and carbon isotope approach used here can be applied to semi-quantitatively estimate spatial and temporal variations of the sources of organic carbon in tropical estuarine and coastal sediments, a major sink for carbon in the ocean.

Reconstructions of the past climate from proxy records involve a wide range of uncertainties at every step of the process. These uncertainties and the subsequent error bar in the reconstruction of a paleoclimatic variable need to be understood and quantified in order to properly interpret the reconstructed variability and to perform meaningful comparisons with climate model outputs. Classic proxy calibration-validation techniques are not well-suited for identifying the causes of reconstruction errors, estimating their relative contribution, or understanding how errors accumulate from a multitude of sources. In this study, we focus on high resolution proxy records based on calcium carbonate geochemistry of sessile organisms such as mollusks, corals, or sclerosponges, and propose an approach based on Monte Carlo simulations with simple numerical surrogate proxies. A freely available algorithm (MoCo, http://www.isem.cnrs.fr/spip.php?rubrique472) is provided for estimating systematic and standard errors of mean temperature, seasonality and variance reconstructed from marine accretionary archive geochemistry. This algorithm is then used for sensitivity experiments in a case study to characterize and quantitatively evaluate the sensitivity of systematic and standard errors to sampling randomness, stochastic uncertainty sources and systematic proxy limitations. The results of the experiments yield an illustrative example of the range of variations that climate reconstruction errors may undergo, and bring to light their complexity. One of the main improvements of this method is the identification and estimation of systematic bias that would not otherwise be detected. It thus offers the possibility of correcting the proxy-based climate from these biases for a more accurate reconstruction. Beyond the findings of error sources for coral and mollusk-based reconstructions, our study demonstrates that numerical simulations based on Monte Carlo analyses are a simple and powerful approach to improve the proxy calibration process. A thourough understanding of the proxy record errors is essential for the interpretation of paleoclimate records from proxies derived from accretionary skeleton geochemistry. The error estimates provided by MoCo are much more comprehensive and therefore closer to reality than error estimates provided by typical calibration studies.

Hydrogen isotope ratios (2H/1H or δ2H) of plant leaf waxes typically covary with those of precipitation, and are therefore used as a proxy for past hydrologic variability. Mangroves present an important exception to this relationship, as salinity can strongly influence 2H fractionation in leaf lipids. To better understand and calibrate this effect, δ2H values of taraxerol and n-alkanes were measured in the leaves of Rhizophora spp. (red mangroves) from three estuaries and four brackish lakes on the Micronesian islands of Pohnpei and Palau, and compared to the δ2H and δ18O values of leaf water, xylem water and surface water. Net 2H discrimination between surface water and taraxerol increased by 0.9 ± 0.2‰ per part per thousand (ppt−1) over a salinity range of 1–34 ppt. Xylem water was always depleted in 2H relative to surface water, and the magnitude of this depletion increased with salinity, which is most likely due to a combination of greater 2H discrimination by roots during water uptake and opportunistic use of freshwater. Changes in the 2H content of xylem water can account for up to 43% of the change in net taraxerol fractionation with salinity. Leaf water isotopes were minimally enriched relative to xylem water and there was not significant variability in leaf water enrichment with salinity, which is consistent with a Peclet-modified Craig–Gordon model of leaf water enrichment. As leaf water enrichment is therefore unlikely to be responsible for increased 2H/1H fractionation in mangrove leaf lipids at elevated salinities, the majority of this signal is most likely explained either by changes in biosynthetic fractionation in response to salt stress or by salinity influenced changes in the timing of water uptake and lipid synthesis.

The TEX86 paleothermometer was applied for reconstructing sea surface temperature (SST) from glycerol dialkyl glycerol tetraethers (GDGTs) in marine sediments. It has become clear that GDGT-producing archaea live throughout the water column, with maximum concentration usually well below the surface mixed layer of the ocean. Why the TEX86 parameter correlates well with SST remains poorly understood. Here we evaluate the fidelity of the TEX86 thermometer using surface sediments and suspended particles from the Yellow Sea (YS), a shallow marginal sea between China and the Korean peninsula. The highest concentration of GDGTs in the water column at a site (A02) from the middle of the YS occurred in the bottom layer, at 70 m. This contrasts with phytoplankton lipids, which were most abundant near the surface. Consistent with the maximum abundance of GDGTs in bottom water, TEX86 values in surface sediments correlated better with mean annual bottom water temperature (BWT, R2 0.81) than with mean annual SST (R2 0.74). Moreover, TEXH86 temperature derived from a global core top calibration gave values 0.1–9.4 °C colder than mean annual SST but much closer to mean annual BWT (2.7–4 °C). Lastly, TEX86 and U 37 K ′ paleotemperature values displayed distinctly different trends over the last 12 kyr for sediments from the shelf between the YS and the East China Sea (ECS), consistent with the notion that TEX86 and U 37 K ′ reflect different temperature signals. This preponderance of evidence supports the use of TEX86 as a proxy for BWT in the YS. Therefore, we propose a local calibration of TEXL86 for reconstructing mean annual BWT (TEXL86 = 0.03 BWT-0.94; R2 0.86, n = 22, P U 37 K ′ ) derived SST with TEXL86 BWT yielded a quantitative reconstruction of the vertical thermal gradient in the YS, and an insight into understanding the impact of the Kuroshio Current and East Asian monsoon on the YS.

Hydrogen isotope ratios ((2) H/(1) H, δ(2) H) of leaf waxes covary with those in precipitation and are therefore a useful paleohydrologic proxy. Mangroves are an exception to this relationship because their δ(2) H values are also influenced by salinity. The mechanisms underlying this response were investigated by measuring leaf lipid δ(2) H and leaf and xylem water δ(2) H and δ(18) O values from three mangrove species over 9.5 months in a subtropical Australian estuary. Net (2) H/(1) H fractionation between surface water and leaf lipids decreased by 0.5-1.0‰ ppt(-1) for n-alkanes and 0.4-0.8‰ ppt(-1) for isoprenoids. Xylem water was (2) H depleted relative to surface water, reflecting (2) H discrimination of 4-10‰ during water uptake at all salinities and opportunistic uptake of freshwater at high salinity. However, leaf water (2) H enrichment relative to estuary water was insensitive to salinity and identical for all species. Therefore, variations in leaf and xylem water δ(2) H values cannot explain the salinity-dependent (2) H depletion in leaf lipids, nor the 30‰ range in leaf lipid δ(2) H values among species. Biochemical changes in direct response to salt stress, such as increased compatible solute production or preferential use of stored carbohydrates, and/or the timing of lipid production and subsequent turnover rates, are more likely causes.

Little is known about the bulk hydrogen stable isotope composition (δ2H) of seaweeds. This study investigated the bulk δ2H in several different seaweed species collected from three different beaches in Brazil, Australia, and Argentina. Here, we show that Ulvophyceae (a group of green algae) had lower δ2H values (between −94? and −130?) than red algae (Florideophyceae), brown algae (Phaeophyceae), and species from the class Bryopsidophyceae (another group of green algae). Overall the latter three groups of seaweeds had δ2H values between −50? and −90?. These findings were similar at the three different geographic locations. Observed differences in δ2H values were probably related to differences in hydrogen (H) metabolism among algal groups, also observed in the δ2H values of their lipids. The marked difference between the δ2H values of Ulvophyecae and those of the other groups could be useful to trace the food source of food webs in coastal rocky shores, to assess the impacts of green tides on coastal ecosystems, and to help clarify aspects of their phylogeny. However, reference materials for seaweed δ2H are required before the full potential of using the δ2H of seaweeds for ecological studies can be exploited. Fil: Carvalho, Matheus C.. Southern Cross University; Australia Fil: Carneiro, Pedro Bastos de Macedo. Universidade Federal do Ceará; Brasil Fil: Dellatorre, Fernando Gaspar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Centro para el Estudio de Sistemas Marinos; Argentina. Universidad Tecnológica Nacional. Facultad Regional Chubut; Argentina Fil: Gibilisco, Pablo Ezequiel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de la Patagonia; Argentina Fil: Sachs, Julian. University of Washington; Estados Unidos Fil: Eyre, Bradley D.. Southern Cross University; Australia

We present a 3000-yr rainfall reconstruction from the Galapagos Islands that is based on paired biomarker records from the sediment of El Junco Lake. Located in the eastern equatorial Pacific, the climate of the Galapagos Islands is governed by movements of the Intertropical Convergence Zone (ITCZ) and the El Nino-Southern Oscillation (ENSO). We use a novel method for reconstructing past ENSO- and ITCZ-related rainfall changes through analysis of molecular and isotopic biomarker records representing several types of plants and algae that grow under differing climatic conditions. We propose that δD values of dinosterol, a sterol produced by dinoflagellates, record changes in mean rainfall in El Junco Lake, while δD values of C34 botryococcene, a hydrocarbon unique to the green alga Botryococcus braunii, record changes in rainfall associated with moderate-to-strong El Nino events. We use these proxies to infer changes in mean rainfall and El Nino-related rainfall over the past 3000 yr. During periods in which the inferred change in El Nino-related rainfall opposed the change in mean rainfall, we infer changes in the amount of ITCZ-related rainfall. Simulations with an idealized isotope hydrology model of El Junco Lake help illustrate the interpretation of these proxy reconstructions. Opposing changes in El Nino- and ITCZ-related rainfall appear to account for several of the largest inferred hydrologic changes in El Junco Lake. We propose that these reconstructions can be used to infer changes in frequency and/or intensity of El Nino events and changes in the position of the ITCZ in the eastern equatorial Pacific over the past 3000 yr. Comparison with El Junco Lake sediment grain size records indicates general agreement of inferred rainfall changes over the late Holocene.

Hydrogen isotope measurements of lipid biomarkers preserved in sediments are most commonly interpreted as qualitative, rather than quantitative indicators of paleoprecipitation owing to an imperfect knowledge of all factors controlling the isotopic fractionation occurring during biosynthesis. Here, we first offer a brief review of appropriate procedures for preparing enriched isotope substrates for use in tracer studies and outline the approximate δD threshold at which this transition occurs. We then present new interpretations to explain deviations from common stable isotope effects observed in our previous culture experiments and other studies. We draw particular attention to the disagreement between intercept and slope for product–substrate relationships from those predicted for isotope systems, even when R2 values are high, and attribute it to kinetic isotope fractionation. We demonstrate that reconstructing paleoenvironmental water δD values by simply adding a Δ to measured biomarkers δD values will result in a bias toward deuterium enriched values. This applies even to implicit reconstructions in the form of qualitative interpretations of measured lipid δD values as indicators of past hydroclimate. We therefore recommend reconstructing water δD values from lipid δD values using fractionation factor (α). We also discuss the apparently contradictory increase in D/H fractionation observed at elevated temperature and suggest that this may be the result of the unique wave-particle duality of hydrogen isotopes, which permits isotopologues to avoid surmounting the activation energy barrier that is necessary in traditional kinetic reactions.

Solvent extracted sediments from El Junco Lake, Galapagos were sequentially hydrolyzed with base and acid. Some of the “bound” lipids thus generated were not detected among the “free” lipids obtained from the initial solvent extract of the freeze dried sediment. These include α-hydroxy acids, β-hydroxy acids, methyl β-hydroxy acids and cutin acids. Other bound lipids differed from the free lipids in their chain length, such as alkanols, sterols, ω-hydroxy acids and (ω-1)-hydroxy acids. A suite of biomarkers with unique ions of m/z 341 were identified in the acid hydrolysates that are most likely C40 cyclic biomarkers derived from the L race of Botryococcus braunii. Free lipids and their isotopic composition are likely to be sufficient for the purpose of paleoenvironmental reconstructions, and therefore base or acid hydrolysis of solvent extracted sediment is not necessary. Nevertheless, the additional lipid biomarkers liberated by base and acid hydrolysis can provide information on microbial inputs to the sediment.

Species composition and vertical distribution of planktonic organisms in Lakes Lalolalo, Lanutavake, and Lano on the Pacific island of Uvea were investigated in relation to physicochemical water column profiles of temperature, conductivity, dissolved oxygen, and pH. The meromictic lakes Lalolalo (maximum depth 88.5 m) and Lanutavake (23.6 m) exhibited a strong chemocline with anoxic conditions below 10 m depth. Mixis was inhibited by sheltered topography, thermal stratification, and in Lalolalo a strong halocline. Crustaceans and rotifers were limited to the oxygenated epilimnion, but diversity and density of ciliates were highest within and below the chemocline. In Lalolalo, euryhaline rotifers dominated the community, reflecting the brackish condition of the lake. Species richness and densities were highest within the shallow lake Lano (3.5 m). On a calm day, hypoxic conditions occurred near the sediment-water interface, but higher oxygen concentrations were observed after heavy winds, indicating occasional mixis. In total, 32 phytoplankton species, 23 ciliate taxa, 18 rotiferan, 1 cladoceran, 1 copepod, and 1 gastrotrich species were identified in the pelagic zones of all three lakes.

Equatorial Pacific SST reconstructions derived from Mg/Ca ratios in planktonic foraminifera Globigerinoides ruber and from alkenone-producing coccolithophorids record different trends throughout the Holocene and the last deglaciation. We set forth the hypothesis that their diverging behavior may be related to different seasonal sensitivities which result from the annually varying production rates of alkenone-producing coccolithophorids and of G. ruber. Using a series of transient paleoclimate model simulations forced with the time-varying forcing history over the last 21 ka, a good qualitative agreement is found between simulated boreal winter temperatures and alkenone-SST reconstructions as well as between simulated boreal summer temperatures and reconstructed Mg/Ca-based SST variations. Pronounced features in the reconstructions that can be readily explained by the conjectured seasonal biases include the mismatch in middle-to-late Holocene temperature trends and the different onsets of deglacial climate change in the eastern equatorial Pacific. The analysis presented here further suggests that through combinations of Mg/Ca and alkenone SST reconstructions information can be gained on annual mean temperature changes and the amplitude of the seasonal cycle in SST. Our study concludes by discussing potential weaknesses of the proposed model-derived seasonal bias interpretation of tropical Pacific SST proxies in terms of present-day core-top data, sediment trap studies, and satellite-based observations of chlorophyll.

A variety of lipid biomarkers were identified in sediments from El Junco Lake, Galapagos and their sources investigated for potential use in paleoclimate applications. A series of unusual sterols was also found, including 4α-methylgorgostanol, reported in only four species of dinoflagellates to date. We also tentatively assigned 22,23-methylene-4α-methyl-24-ethylcholest-5-en-3β-ol, the mass spectrum of which matched a sterol found in resting cysts of the dinoflagellate Peridinium umbonatum. In addition, we identified the novel sterol 4α,22,23,24-tetramethyl-5α-cholest-22E-en-3β-ol. Based on the unique sterol distribution, we hypothesize that a dinoflagellate from the genus Peridinium was the primary source of dinosterol and the novel sterols throughout the sediment record. The source specificity and abundance throughout the 3.7 m of recovered sediment make dinosterol an excellent target for hydrogen isotope analysis for use as a paleohydrological proxy in future studies. The abundant C30 and C32 1,ω20-diols and keto-ols, C29 9,10-diol and C29 1,ω9,ω10-triol likely derive from the ferns Azolla microphylla and Cyathea weatherbyana, while sources of the C30 1,ω16-diol and keto-ol, C32 1,ω18-diol and keto-ol, and the C30–C32 n-alken-1-ols are likely limited to aquatic microalgae. Due to their source specificity, these diol, triol, keto-ol, and n-alkenol biomarkers present further tools for studying past environmental and climatic change.

We present a preliminary assessment of the hydrogen isotopic composition of individual alkenones (C 37–39 di-, tri- and tetra-unsaturated methyl- and ethyl-ketones) purified from surface lake sediment from sites spanning a range of salinities from 20 to 133 ppt. Combining these measurements with measurements of the hydrogen isotopic composition of modern lake water, we estimate the magnitude of D/H fractionation across our sample set and observe reduced sensitivity to salinity compared to observations from Emiliania huxleyi and Gephyrocapsa oceanica . This lends support to the hypothesis that D/H fractionation during biosynthesis is less sensitive to changes in salinity in alkenone producers from continental interior sites than in producers from open marine environments. We also observe stronger correlations between the tetra-unsaturated alkenone δD values with water δD values, and associated α values with salinity than we do for the di- and tri-unsaturated compounds, and suggest that this may result from increased number of lacustrine haptophyte species producing the di- and tri-unsaturated as compared to the tetra-unsaturated.

Measurements of net ecosystem productivity (NEP) and calcification (NEC) from contemporary coral reefs provide a baseline for monitoring the impacts of future stressors like ocean acidification and sea-surface warming. However, separating secular trends from natural variability requires NEP and NEC records across a wide range of spatial and temporal scales. One promising way to make these measurements is with autonomous pH and O2 sensors. Crucially, the accuracy of this approach relies on knowledge of the in situ ecosystem productivity quotient (Q), which indicates the moles of O2 consumed per mole CO2 produced. Using co-located measurements of total dissolved inorganic carbon (DIC), total alkalinity, and dissolved O2, we empirically determined Q during a three-year field campaign on Tetiaroa Atoll, French Polynesia. Empirical values of daily net Q (−1.02 ≤ Qnet ≤ −0.47) frequently differed from both the canonical value of −1.45 for the ocean and the value of −1 often assumed for tropical reef ecosystems. Furthermore, Q changed on hourly timescales, and integrated daily values differed between days. We hypothesize that captive bubbles on the surfaces of coral, macroalgae and other substrates can explain these variations in Q, with other influential mechanisms being mixing between parcels of water, sedimentary denitrification, and ammonium-fueled primary productivity. Our findings, which are robust to changes in the model that is used to correct for advection and gas exchange, as well as changes in model parameters, suggest that future investigations of metabolism on reefs should be based on measurements of carbonate chemistry variability, rather than O2 evolution. Otherwise, large biases in NEP and NEC could result (>40%).

The T E X 86 H (or TEX86) paleothermometer has been widely applied for reconstructing water temperature in marine settings, particularly in marginal seas which often archive paleoclimate records with high temporal resolution. Yet debate remains regarding the water depth at which the T E X 86 H signal is imparted in these settings. In this study, East China Sea (ECS) surface sediments and suspended particulate matter (SPM) from different water depths (0–60 m) were analyzed for lipid biomarkers to better constrain the depth to which the sedimentary T E X 86 H signal corresponds. Our data showed divergent vertical distributions between archaeal lipids and phytoplankton lipids. The maximum concentration of core lipid isoprenoid glycerol dialkyl glycerol tetraethers (core iGDGTs) appeared in bottom waters (2–5 m above the sea floor) at all sites, whereas phytoplankton lipids showed highest abundance in surface waters (0 m). Consistent with the maximum core iGDGTs concentration in bottom waters, the best correlation was observed between SPM T E X 86 H values and 2–4 weeks averaged water temperatures from the bottom waters. Moreover, the SPM core iGDGTs content was higher in bottom waters than in surface sediments, implying that in situ production rather than sediment resuspension leads to the maximum concentration of core iGDGTs in bottom waters. By compiling published surface sediment data from water depths T E X 86 H with bottom water temperature (BWT) exceeds that with sea surface temperature (SST). Therefore, it is proposed that sedimentary T E X 86 H is a BWT proxy in the shallow ECS.