Alkenone Concentrations Research Articles

Pliocene Weakening of Gradients in Temperature but Not in Productivity in the Eastern Equatorial Pacific

AbstractThe modern eastern equatorial Pacific Ocean (EEP) exhibits strong upwelling, producing pronounced gradients in sea surface temperature (SST), nutrient concentration, and biological productivity between 80° and 140°W. During the globally warmer late Pliocene (3.0–3.6 Ma), the EEP may have experienced permanent El Niño‐like conditions, supported by a reduced SST gradient across the equatorial Pacific. However, the weakened east‐west SST gradient has been controversial, with disparate results depending on the proxy used to monitor Western Warm Pool SSTs. We present new Pliocene alkenone‐based SST and paleoproductivity records from four Ocean Drilling Program (ODP) cores spanning an east‐west transect across the EEP, which present an internally consistent picture of SST and productivity gradients in the modern cold tongue, resolved at orbital‐scale variability. Strong agreement between core top reconstructions and satellite estimates indicates that alkenone paleotemperature and paleoproductivity proxies are appropriate for reconstructing Pliocene EEP conditions. The average SST gradient between 90° and 120°W was reduced from the modern 1.8°C gradient to 0.9°C in the late Pliocene. Despite the weakened SST gradient, the surface productivity gradient was stronger during the late Pliocene compared to modern, based on calibrated X‐ray fluorescence biogenic opal and alkenone average accumulation rates. Contrary to modern El Niño SST and productivity patterns, reduced Pliocene surface productivity did not accompany the weakened SST gradient. Instead, strong Pliocene biogenic opal and alkenone concentration accumulation gradients in the eastern EEP suggest that subsurface tilting of the nutricline and thermocline persisted to supply vigorous upwelling of warm but nutrient‐rich subsurface waters in a warmer climate.

Polyunsaturated long-chain alkenones in the lake sediments of North-Minusinsk valley (southern Siberia) as a paleo-indicator of climate change

The research of biochemical thermo- and salt-sensitive markers - long-chain alkenones in sediments of lakes of the North Minusinsk valley is an important contribution to the understanding of climatic changes in this region. We found that the total concentration of alkenones in the top layers of bottom sediments of saline lakes increases significantly at a salinity of about 20 g l-1. Also, it was found that the average chain length of alkenones in the surface sediments rises with increasing salinity and thus can be used as an indicator of salinity, and the ratio of alkenones C37/C38 varies widely, suggesting that this index can be used as a paleo-indicator of salinity. The high correlation between ,  and  indices with salinity indicates their potential for paleosalinity reconstruction. The obtained functional dependences were additionally verified in the core of bottom sediments of Lake Utichie-3 and confirmed the possibility of using alkenones as a paleo-indicator of climatically determined salinity changes in endorheic lakes. The taxonomic composition of haptophyte algae and alkenone composition in the studied lakes were comparable to lakes in prairie regions of North America.

Impact of saponification and silver-nitrate purification on lacustrine alkenone distributions and alkenone-based indices

A growing interest in lacustrine alkenones as a proxy for continental paleotemperature reconstructions accompanied important methodological improvements over the past two decades. New gas chromatography (GC) columns were used for alkenone analysis, that drastically improved alkenone separation, especially for freshwater lakes. However, these recent advances are sometimes not sufficient in separating compounds that interfere with alkenones in the resulting chromatograms and concurrently, new chemical procedures were implemented to further clean up the samples. Here we investigate the impact of two clean-up procedures, saponification and silver-nitrate purification, on alkenone distribution, alkenone quantification, and C37 alkenone-based indices, including the U37K index. The silver-nitrate purification modified the C37 alkenone distribution and thus the C37 alkenone-based indices, especially the U37K index, in 6 out of the 9 studied samples by further retaining alkenones with more double bonds. These changes would result on an average error of 3 °C in reconstructed temperatures. Saponification also influenced the C37 alkenone distribution mainly by removing co-eluting compounds, thereby improving the quality of the results. Both saponification and purification resulted in the reduction of the C37 alkenone concentration by almost half. Clean-up steps should thus be used carefully, paying particular attention to any change in alkenone distribution and concentration. Limiting the use of additional clean-up steps reduces the risk of modifying the alkenone distribution.

Anthropogenic acidification of surface waters drives decreased biogenic calcification in the Mediterranean Sea

Anthropogenic carbon dioxide emissions directly or indirectly drive ocean acidification, warming and enhanced stratification. The combined effects of these processes on marine planktic calcifiers at decadal to centennial timescales are poorly understood. Here, we analyze size normalized planktic foraminiferal shell weight, shell geochemistry, and supporting proxies from 3 sediment cores in the Mediterranean Sea spanning several centuries. Our results allow us to investigate the response of surface-dwelling planktic foraminifera to increases in atmospheric carbon dioxide. We find that increased anthropogenic carbon dioxide levels led to basin wide reductions in size normalized weights by modulating foraminiferal calcification. Carbon (δ13C) and boron (δ11B) isotopic compositions also indicate the increasing influence of fossil fuel derived carbon dioxide and decreasing pH, respectively. Alkenone concentrations and test accumulation rates indicate that warming and changes in biological productivity are insufficient to offset acidification effects. We suggest that further increases in atmospheric carbon dioxide will drive ongoing reductions in marine biogenic calcification in the Mediterranean Sea.

Alkenone Uk’37 index differs between thermally separated populations of fin whales and krill

Although habitat temperature is an important determinant of species distributions, its measurement is difficult in mobile marine organisms. Chemical tracers such as alkenones constitute an alternative tool for gauging marine habitat temperature, something particularly relevant in the current scenario of global warming. The unsaturation index of alkenones, the Uk’37 index, can be used to estimate sea surface temperature (SST) in living organisms. Here, we analyse alkenone concentrations in a predator species, the fin whale, and its prey, the northern krill, both sampled in 2 areas with different SSTs: SW Iceland and NW Spain. In NW Spain (but not in SW Iceland), alkenone concentrations were higher in krill than in fin whale blubber, suggesting that they may biodilute along the trophic transfer. Consistently with local SST, the Uk’37 index was lower in the Icelandic samples of both fin whales and krill than in those from NW Spain, confirming the ability of this index to distinguish populations that are thermally separated. Whales are migratory animals; thus, the alkenones present in their blubber are a mix of those ingested locally and in recently visited regions. Consistently, the Uk’37 index showed a higher correlation with the local SST in krill than in fin whales. Alkenone turnover rates should be considered when using this proxy to assess the thermal range of the feeding habitat of a species. We conclude that the Uk’37 index is a potentially efficient tool to study the behaviour of whales, thus providing an alternative perspective to other chemical tracers.

Long-chain alkenones in the lake sediments of North-Minusinsk Valley (southern Siberia): implications for paleoclimate reconstructions

Long-chain alkenones (LCAs) are polyunsaturated methyl or ethyl ketones exclusively produced by Groups 1–3 Isochrysidales, an order of haptophyte algae. LCAs have great potential as temperature and salinity proxies for paleo-reconstructions. Alkenone producers occur extensively in lacustrine and in open marine systems. However, factors influencing LCAs occurrence, abundance, compositional pattern, and unsaturation indices are insufficiently understood in lakes of southern Siberia. Here we investigated alkenone distributions in freshwater to saline lakes of a semi-arid region of southern Siberia (n = 22). We find that total alkenone concentrations increase dramatically above a salinity of about 20 g L−1. Analysis of 18S rRNA and the alkenone composition suggest that all alkenone-containing lakes are inhabited by Group 2 haptophytes except one low-saline lake, where Group 1 representatives prevail. The C37/C38 ratio is negatively correlated with salinity, and, hence, this parameter can be used as a salinity indicator. At the same time, %C37:4 is not correlated with salinity, whereas the average chain length of alkenones increases with salinity. The taxonomic composition of haptophytes and the alkenone composition were comparable to those in the lakes in the Canadian Prairies, potentially because of the similarities in climatic factors and ionic composition of lake water in the two regions. This study shows for the first time that unsaturation indexes U40K and U40K′ are positively correlated with salinity and, thus, can be used as a salinity proxy for regional climate reconstruction.

Distribution characteristics of terrestrial and marine lipid biomarkers in surface sediment and their implication for the provenance and palaeoceanographic application in the northern South China Sea

Multi biolipids, including plant wax, algal biomarkers, and glycerol dialkyl glycerol tetraethers (GDGTs), were studied in surface sediments of the northern South China Sea (SCS). Unusual elevated contents of odd n-alkanes (C27, C29, C31) and even n-alkanols (C22, C24, C26) derived from terrestrial plant wax in the outer shelf areas indicated the strong influence of deep-water counter current in redistributing and deposition of Taiwan Island sourced organic matter (OM) in the northern SCS. The spatial distribution of dinosterol and brassicasterol (corresponding to dinoflagellates and diatoms of high competitive advantage for nutrients), respectively, displayed a high abundance near the Pearl River Estuary (PRE), southwest off Taiwan Island, and southeast off Hainan Island, in contrast to the relatively high biomass of C37-alkenones, C30-diol/ketol and isoprenoid GDGTs (iGDGTs) in the outer shelf oligotrophic areas, indicating their feasibility in marine productivity reconstruction. The observed high contents of alkenones than brassicasterol in the sediment, albeit the overall dominance of diatoms in the SCS surface water, showed differences in the degradation and/or preservation efficiency of different organic lipids in sediments. The influences of season, source organisms, and different regression methods on the sea surface temperatures (SST) reconstruction were also evaluated in the region extending from shelf to deep water. The reconstructed SSTs based on U37K' mainly reflected spring SSTs, indicating the influence of growing seasons on the source organism. The reconstructed TEX86 SST corresponded more with the mean annual SST in the northern SCS, mainly in the deep-water regions. Changes in the total composition of the archaeal community and population is suggested to be the major factor causing the unusually low and wide range of TEX86 SSTs in the nearshore areas. The combination of the two organic paleothermometers offers complementary information on SST developments in the northern SCS. Multi-biomarkers study in the present work indicates the feasibility of lipids in paleoecology and paleoclimate reconstruction. However, care must be taken in palaeoceanographic reconstruction due to complex hydrological and biological influence, i.e., the deep lateral current on the continental matter transportation, the differential degradation of various organic lipids, and the different seasonal and/or source organism community bias on the organic thermometer in the study area.

Paleoproductivity proxies and alkenone precursors in the Western Mediterranean during the Early-Middle Pleistocene transition

Multidisciplinary analyses (taxonomic analysis of coccolithophore assemblages, stable oxygen isotopes, marine and terrestrial biomarkers) have been carried out on sediments from Ocean Drilling Program Site 975 in the Algero-Balearic basin, through late marine isotope stage (MIS) 20-19 (800–756 ka). The aim is to compare coccolithophore paleoproductivity proxies, such as C37 alkenone concentration and nannofossil accumulation rate (NAR), and understand their relationship with paleoceanographic condition and paleoenvironmental changes, alkenone-producing precursors and unsaturated C37 alkenone compounds. The patterns of C37 alkenones and NAR provide reliable information on past paleoproductivity changes since coccolith dissolution and organic matter preservation were excluded as relevant processes at the site. This is testified by the high values of Nannofossil Dissolution Index and relation between C37 alkenone concentration and Alcohol Preservation Index, the latter used as a proxy of sea bottom ventilation in the basin. A weak mismatching between NAR and C37 alkenone concentration records has been observed and related to paleoenvironmental factors and ecological preferences of alkenone-producing species. Temperature variations mostly controlled the alternating interspecific abundance variations of these taxa through glacial-interglacial and stadial-interstadial climate phases. The percentage abundances of alkenone-producing species, Gephyrocapsa caribbeanica and Gephyrocapsa with open central area (mainly G. margerelii-G. muellerae) strongly co-varied with the percentages of C37:2 and C37:3, respectively during warm and cool-cold periods, suggesting their prominent role in producing these unsaturated C37 alkenone compounds. Moreover, Gephyrocapsa spp. with open central area > 3 μm were likely the main C37:4 producers during the colder late MIS 20 stadial and stadial phases. Other factors in addition to temperature influenced the paleoproductivity proxy patterns. The oceanographic condition established during MIS 20-MIS 19 deglaciation and the more nutrient-rich surface waters during the orbitally-controlled organic-rich layer deposition in the early MIS 19 enhanced primary productivity leading to higher production and preservation of total C37 alkenones.

Glacial isostatic adjustment near the center of the former Patagonian Ice Sheet (48°S) during the last 16.5 kyr

Our understanding of glacial isostatic rebound across Patagonia is highly limited, despite its importance to constrain past ice volume estimates and better comprehend relative sea-level variations. With this in mind, our research objective is to reconstruct the magnitude and rate of Late Glacial and Holocene glacial isostatic adjustment near the center of the former Patagonian Ice Sheet. We focus on Larenas Bay (48°S; 1.26 km2), which is connected to Baker Channel through a shallow (7.4 m) and narrow (ca. 150 m across) inlet, and hence has the potential to record periods of basin isolation and marine ingression. The paleoenvironmental evolution of the bay was investigated through a sedimentological analysis of a 9.2 m long radiocarbon-dated sediment core covering the last 16.8 kyr. Salinity indicators, including diatom paleoecology, alkenone concentrations and CaCO3 content, were used to reconstruct the bay's connectivity to the fjord. Results indicate that Larenas Bay was a marine environment before 16.5 cal kyr BP and after 9.1 cal kyr BP, but that it was disconnected from Baker Channel in-between. We infer that the postglacial rebound started before 16.5 cal kyr BP and that it outpaced global sea-level rise until slightly before 9.1 cal kyr BP. During the Late Glacial and early Holocene, the center of the former Patagonian Ice Sheet experienced an absolute uplift of ca. 96 m, at an average rate of 1.3 cm/yr. During the remainder of the Holocene, glacial isostatic adjustment continued (ca. 20 m), but at a slower average pace of 0.2 cm/yr. Comparisons between multi-millennial variations in the salinity indicators and existing records of global sea-level rise suggest that the glacial isostatic adjustment rate also fluctuated within these time intervals, likely in response to glacier dynamics. More specifically, most of the glacial isostatic adjustment registered between 16.5 – 9.1 cal kyr BP seems to have occurred before meltwater pulse 1A (14.5 – 14.0 kyr BP). Likewise, it appears that the highest glacial isostatic rebound rates of the last 9.1 kyr occurred during the late Holocene, most likely in response to glacier recession from their Neoglacial maxima. This implies a relatively rapid response of the local solid earth to ice unloading, which agrees with independent modelling studies investigating contemporary uplift. We conclude that the center of the former Patagonian Ice Sheet experienced a glacial isostatic adjustment of ca. 116 m over the last 16.5 kyr, and that >80% occurred during the Late Glacial and early Holocene.

A 1‐Million‐Year Record of Environmental Change in the Central Mediterranean Sea From Organic Molecular Proxies

AbstractThe Mediterranean Sea is particularly sensitive to climate oscillations and represents a key location to study past climatic and oceanographic changes. One valuable source of paleoceanographic information is through molecular biomarkers in deep sea sediments. This approach has been applied in a number of studies in this basin, but only covering the most recent glacial/interglacial cycles. Here we present, for the first time in the Mediterranean Sea, a molecular biomarker record from the Strait of Sicily that covers the last million years until the present, almost continuously. We present data on alkenone derived index sea surface temperatures (SST) and provide insights on the evolution of the phytoplankton community composition and terrestrial inputs through the analysis of the concentrations of alkenones, brassicasterol and long‐chain alcohols. The ‐SST record followed a climatic evolution modulated by glacial/interglacial cycles with a marked increase in the 100 kyr‐amplitude of the glacial cycles at ∼430 ka, coincident with the Mid‐Brunhes transition. In addition, SSTs were consistently higher compared with other records in the western Mediterranean, indicative of the progressive warming that surface waters experience along their transit from the Strait of Gibraltar to the Central Mediterranean. Regarding the concentrations of alkenones and brassicasterol, they displayed distinct alternate peaks, some of them coeval with the deposition of sapropels. This suggests that different environmental and oceanographic conditions characterized each sapropel which, together with changes in terrestrial inputs and the degree of oligotrophy, induced the alternate proliferation of coccolithophores and diatoms.

Carbonate dissolution and environmental parameters govern coccolith vs. alkenone abundances in surface sediments from the northwest North Atlantic

We determined the abundances and concentrations of coccoliths and alkenones in 66 surface sediment samples from the northwest North Atlantic to evaluate the role that surface ocean temperature, salinity, sea-ice cover, and productivity have on the regional distribution of these two biological remains produced by haptophytes in the photic zone. In areas with sea-ice cover of more than 1 month per year, coccolith and alkenone concentrations in sediments are extremely low to nil. Elsewhere, the distribution of coccolith taxa generally displays strong relationships to water temperature, salinity, and productivity. Coccolithus pelagicus is associated with low summer sea-surface temperatures (<8°C) and relatively high summer sea-surface salinities (>33.5), whereas Helicosphaera carteri seems to follow the path of the North Atlantic Drift. The distribution of Emiliania huxleyi, the dominant alkenone producer, is not strongly correlated with that of alkenones. Calcite dissolution in shelf sediments could explain the occurrence of alkenones in the absence of coccoliths but alkenone production by non-calcifying haptophytes seems to also exert some control on alkenone concentrations in surface sediments, thus blurring alkenone abundance links to coccolithophorid production and their relative preservation.

Biogenic carbonate fluxes and preservation in the northwestern Labrador Sea since the Last Glacial Maximum

The important changes that took place in the glacial cycle at the termination, from the Last Glacial Maximum to the present interglacial, deserve an examination of ocean sedimentary records that document past productivity, carbon fluxes, and carbonate preservation. In this study, we analyzed coccoliths, alkenones, and foraminifers in core HU2008–029-004 PC (61.46°N and 58.04°W, water depth = 2,674 m) from the northwestern Labrador Sea to document linkages between hydrographic conditions, biogenic carbonate fluxes to the seafloor, and their preservation/dissolution during the last 25,000 years. Large changes in coccolith and foraminifer concentrations are recorded, with sediments from the last glacial interval containing significantly less carbonate microfossils (9.5 ± 3.9 × 105 coccoliths g−1 and 2,860 ± 580 planktonic foraminifers g−1) than sediments from the deglacial and postglacial intervals (up to 3.1 × 108 coccoliths g−1 and 2.9 × 104 foraminifers g−1). Three foraminifer-based calcite dissolution indices were used to evaluate biogenic carbonate preservation: the planktonic foraminifer fragmentation index, the ratio of benthic-to-planktonic foraminifers (B/P), and the ratio of organic linings to benthic foraminifers (OL/B). Fragmentation remained low throughout the postglacial (mean of 4%) but reached up to 8% in the deglacial and peaked at 16% in samples from the Bølling-Allerød of the late glacial interval. Samples from the Bølling-Allerød and the deglacial interval also display a slightly elevated B/P index (>0.15), which suggests that some dissolution may have occurred. In contrast, with the exception of the Bølling-Allerød and the deglacial interval, near zero OL/B values characterize most of the sequence, suggesting good biogenic carbonate preservation, which implies that the low biogenic carbonate and coccolith content in sediments of the glacial stage mirror low productivity of calcifying organisms. The elevated fragmentation of foraminifers during the Bølling-Allerød and the deglacial interval, a time of elevated productivity and low percentages of ice-rafted debris, may indicate the development of calcite undersaturated porewaters and consequent dissolution resulting from oxic remineralization of sedimentary organic matter. We also identify a significant decoupling of coccolith and alkenone concentrations throughout the core. Colder-than-expected UK37-SST estimates from the alkenones of the glacial interval rule out possible allochthonous inputs from lower-latitude locations. Instead, our records imply that at least during the glacial interval, alkenones were produced by non-calcifying haptophytes that may not follow the canonical UK37-based temperature calibrations.

Control Mechanisms of Primary Productivity Revealed by Calcareous Nannoplankton From Marine Isotope Stages 12 to 9 at the Shackleton Site (IODP Site U1385)

AbstractNowadays, primary productivity variations at the SW Iberian Margin (IbM) are primarily controlled by wind‐driven upwelling. Thus, major changes in atmospheric circulation and wind regimes between the Marine Isotope Stages (MIS) 12 and 9 could have driven substantial changes in phytoplankton productivity which remains poorly understood. We present a high‐resolution calcareous nannofossil record from the Shackleton Site Integrated Ocean Discovery Program Site U1385 that allow the assessment of primary productivity and changing surface conditions on orbital and suborbital timescales over the SW IbM. These records are directly compared and integrated with terrestrial – Mediterranean forest pollen – and marine – benthic and planktic oxygen stable isotopes (δ18O), alkenone concentration [C37], Uk´37‐Sea Surface Temperature and % C37:4 – proxy records from Site U1385. Our results indicate intra‐interglacial increase in primary productivity together with intensification of the Azores anticyclonic high‐pressure cell beyond the summer that suggests a two‐phase upwelling behavior during the full interglacial MIS 11c (∼420–397 ka), potentially drived by atmospheric NAO‐like variability. Primary productivity is largely enhanced during the inception of glacial MIS 10 and the early MIS 10 (∼392–356 ka), linked to intensified upwelling and associated processes during a period of strengthened atmospheric circulation. In agreement with the conditions observed during Heinrich events of the last glacial cycle, primary productivity reductions during abrupt cold episodes, including the Heinrich‐type (Ht) events 4 to 1 (∼436, 392, 384 and 339 ka) and the Terminations V and IV, seems to be the result of halocline formation induced by meltwater arrival, reducing the regional upward nutrient transference.

Unusual Shorter‐Chain C35 and C36 Alkenones from Commercially Grown Isochrysis sp. Microalgae

AbstractUnusual shorter‐chain alkenones (C35 and C36) were the dominant alkenones (rather than C37 and C38) isolated from commercially grown Tisochrysis lutea microalgae. Structures of the alkenones were determined by analyzing the alkenones and their products following a butenolysis reaction by comprehensive two‐dimensional gas chromatography coupled to a high‐resolution time‐of‐flight mass spectrometer (GC × GC‐TOF HRMS). The major alkenones were identified as (15E,22E)‐heptatriaconta‐15,22‐dien‐2‐one and (16E,23E)‐octatriaconta‐16,23‐dien‐3‐one, similar to one previous finding of abundant shorter‐chain alkenones in a culture of Emiliania huxleyi CCMP2758, yet distinguishing them from C35 and C36 alkenones extracted from Black Sea sediments with different double bond positions. Our data suggest a possible shared trigger (e.g. nutrient stress) that may occur during cultivation resulting in shorter‐chain alkenones from both the aforementioned E. huxleyi and T. lutea. Moreover, for T. lutea, short‐chain alkenone biosynthesis can be accompanied by lower alkenone content but higher amounts of valuable docosahexaenoic acid (DHA). These results, therefore, extend beyond alkenones and are important to established and emerging algal oil‐based industries.

Ecolosystem changes and possible mechanisms of Dagze Co in the Tibetan Plateau during the past 1000 years

在全球变化的背景下，厘清湖泊生态系统对气候环境以及人类活动的响应机制对制定社会的适应政策非常重要.目前的研究手段如现场观测和围隔实验等可以很好地揭示湖泊生态系统在有观测记录以来的演替和变化过程，但是不能提供历史时期湖泊生态系统的变化及其对气候环境变化和人类活动的响应.古湖沼学可以为探讨湖泊生态系统的长期变化及其对气候环境变化的响应提供重要信息.本文以青藏高原中部无鱼湖泊达则错为研究对象，利用沉积物岩芯西藏拟溞（Daphnia tibetana）残体丰度和总烯酮含量重建该区过去1000年的浮游生物记录；利用总氮、总磷以及总有机碳含量重建过去1000年湖泊营养盐以及有机质变化记录；结合烯酮不饱和度重建的古温度记录，探讨达则错过去1000年生态系统变化及其对气候环境演变的响应机制.研究发现达则错湖泊生态系统尤其是生产力在自然状况和人类活动影响下存在显著变化.在自然状况下，较高的湖泊初级和次级生产力发生在温度较低和湖水营养盐浓度较高时；而在过去150年，达则错湖泊环境受到人类活动影响显著，湖泊生产力发生相应变化，较高的湖泊生产力发生在温度较高时期，其主要受由人类活动带来的营养盐元素浓度控制.研究结果表明达则错湖泊生态系统在人类活动影响下发生了显著的改变.;Understanding the evolution of the lake ecosystem and their response to climate change and human activities on the long-term scale is important for making social adaptation policies. Current research methods, such as in-situ observations and enclosure experiment, can well reveal the succession and change process of lake ecosystem during the past few decades, but cannot provide the change of lake ecosystem or its response to climate change and human activities in the historical period. Paleolimnology can provide valuable information for the study of long-term changes of lake ecosystem and its response to climate and environment changes. Here, we took Dagze Co, a fishless lake in the central Tibetan Plateau as the research object. Firstly, Daphnia tibetana remains abundance, and total alkenone content in the lake sediment cores were analysed to reconstruct the plankton records of the past 1000 years. Then, total nitrogen, total phosphorus and total organic carbon were analysed to reconstruct the nutrients and organic matter records of the lake during the past 1000 years. Lastly, paleotemperature record reconstructed by the unsaturation index of long-chain alkenones in the sediment core of Dagze Co obtained in previous work were combined to study the ecosystem changes and their response mechanisms to the evolution of climate and environment in the past 1000 years. The results show that the lake ecosystem, especially lake productivity, has significant changes under the influence of natural conditions and human activities. Under the natural conditions, the higher primary and secondary productivity of the lake occurs when the temperature is lower, and the nutrient content of the lake is higher. However, in the past 150 years, the lake environment was significantly affected by human activities, and lake productivity changed accordingly. The higher lake productivity occurred in the period of high temperature, which was mainly controlled by the nutrient element content brought by human activities. The results show that the lake ecosystem has changed significantly under the influence of human activities.

Appraisal of sedimentary alkenones for the quantitative reconstruction of phytoplankton biomass

Marine primary productivity (PP) is the driving factor in the global marine carbon cycle. Its reconstruction in past climates relies on biogeochemical proxies that are not considered to provide an unequivocal signal. These are often based on the water column flux of biogenic components to sediments (organic carbon, biogenic opal, biomarkers), although other factors than productivity are posited to control the sedimentary contents of the components, and their flux is related to the fraction of export production buried in sediments. Moreover, most flux proxies have not been globally appraised. Here, we assess a proxy to quantify past phytoplankton biomass by correlating the concentration of C37 alkenones in a global suite of core-top sediments with sea surface chlorophyll-a (SSchla) estimates over the last 20 y. SSchla is the central metric to calculate phytoplankton biomass and is directly related to PP. We show that the global spatial distribution of sedimentary alkenones is primarily correlated to SSchla rather than diagenetic factors such as the oxygen concentration in bottom waters, which challenges previous assumptions on the role of preservation on driving concentrations of sedimentary organic compounds. Moreover, our results suggest that the rate of global carbon export to sediments is not regionally constrained, and that alkenones producers play a dominant role in the global export of carbon buried in the seafloor. This study shows the potential of using sedimentary alkenones to estimate past phytoplankton biomass, which in turn can be used to infer past PP in the global ocean.

Comparison of Late Neogene Uk′37 and TEX86 Paleotemperature Records From the Eastern Equatorial Pacific at Orbital Resolution

AbstractKey features of late Neogene climate remain uncertain due to conflicting records derived from different sea surface temperature (SST) proxies. To understand scenarios in which proxy‐derived temperature estimates can be used interchangeably or are instead measuring different aspects of the same system, it is necessary to explore both the consistencies and differences between specific paleothermometers. Here, we report orbital‐scale climate records from ODP Site 846 in the eastern equatorial Pacific (EEP) for the interval from ~5–6 Ma using alkenone and archaeal lipid paleothermometers. Results from both proxies are similar in their secular trends and magnitude of long‐term temperature change, and spectral analysis demonstrates that the records are coherent and in‐phase in both the obliquity and precession bands. However, we find that the temperatures reconstructed by TEX86 are consistently offset toward colder values by ~2 °C relative to Uk′37‐derived temperatures in global calibrations, or by ~0.8 °C in Bayesian‐based calibrations. All combinations of calibrations also yield approximately twice the amplitude of orbital‐scale variation in TEX86 relative to Uk′37‐derived temperature fluctuations. Both temperature proxies are negatively correlated with sedimentary alkenone concentrations, which we use as an indicator of increased export productivity. Removing this productivity contribution from TEX86 results in an adjusted TEX86 record with temperature sensitivity identical to Uk′37. In future applications, this signal may be decoupled using additional sedimentary indicators of paleoproductivity, which likely will be most important for upwelling zone environments. There remain other nonexplained factors that contribute to differences between TEX86 and Uk′37 that warrant additional investigation.

Paleoenvironmental and paleoclimatic variations around Lake Van (Eastern Turkey) recorded by sedimentary source specific biomarkers 250–130 ka (MIS7 and MIS6)

Paleoclimatic changes during MIS7 and MIS6 remain poorly described in the Near East. We quantified source-specific biomarkers in Lake Van sediments during the interglacial/glacial cycle MIS7/MIS6. Long-chain n-alkanes produced by land-derived vegetation, as well as long-chain alkenones and sterols (namely brassicasterol and dinosterol) produced by aquatic algae were investigated. Stable hydrogen isotopic measurements (δ2H) on n-C29 alkanes were used as a proxy for aridity and revealed three wetter periods interrupted by two drier intervals during MIS7. In contrast, during the MIS6 glaciation, a generally drier climate was predominant. During the warmer and wetter periods of MIS7, a higher input of aquatic organic matter to Lake Van sediments was recorded by higher concentrations of long-chain alkenones, dinosterol and brassicasterol. Long-chain alkane abundances do not show a pattern related to aridity and were observed in higher concentrations in wetter as well as drier periods. Generally, in the Eastern Mediterranean, a wetter interglacial interrupted by drier episodes followed by a dry glacial period was the common feature observed during the MIS7/MIS6 interglacial/glacial cycle. However, in comparison to the last interglacial/glacial cycle the extreme dry glacial period registered around Lake Van from MIS5d to MIS2 was apparently unique and not equaled by a similar event within the last 250 ka.

Sea-surface temperature, productivity and hydrological changes in the Northern Indian Ocean (Maldives) during the interval ~575-175 ka (MIS 14 to 7)

The South Asian Monsoon (SAM) drives seasonal changes in the atmospheric and ocean circulation of the tropical Indian Ocean, affecting precipitation on land and oceanic primary productivity. This work examined sediments from the International Ocean Discover Program (IODP) Site U1467 (IODP Expedition 359) located in the Maldives Inner Sea (Northern Indian Ocean) at a water depth of 487 m. The Maldives Inner Sea is a perfect location to study past changes in tropical climate and ocean circulation related to monsoon dynamics in the Equatorial Indian Ocean. This study focuses on the ~575-175 ka interval, from Marine Isotope Stage (MIS) 14 to 7, an important period for human evolution and dispersal to Eurasia.We reconstructed sea-surface temperature (SST), based on alkenone unsaturation index (U37K'), hydrological changes, based on terrestrial input of n-alkanes, and past surface ocean productivity, based on total C37 alkenones concentration. The U37K' -SST record shows a difference of about 1.5 °C between glacial and interglacial periods, clearly showing all stages and substages from MIS 14 to 7, and revealing a connection between ice sheets extension and SST at the equatorial region. The n-alkanes concentration and average chain length index indicate vegetation changes at the Indian Peninsula with drier conditions generally associated with glacial periods. Precipitation increases abruptly at the end of terminations, lasting for a variable time interval in each interglacial period, except for MIS 13. However, other mechanisms superimposed to the glacial-interglacial forcing, such as the Indian Ocean Dipole (IOD) mode, have been invoked to explain shorter-scale variability in precipitation over India. The total alkenone concentration record indicates that primary productivity at Site U1467 is strongly associated with orbital changes, probably related to the summer inter-tropical insolation gradient (SITIG, 23N-23S on June 21st). High primary productivity occurred during intervals of low SITIG, which resulted in strong inter-monsoon (April-May and October-November) Indian Ocean Equatorial Westerlies (IEW) and reduced precipitation in the equatorial region. This mechanism may also be related to the IOD, which affects the strength of the IEW.

Air-sea interactive forcing on phytoplankton productivity and community structure changes in the East China Sea during the Holocene

Abstract Phytoplankton productivity and community structure in the East China Sea (ECS) play an important role in marine ecology and carbon cycle, but both have been changing rapidly in response to recent oceanic and atmospheric circulation changes. However, the lack of long-term records of phytoplankton productivity and community structure variability in the region hinders our understanding of natural forcing mechanisms. Here, we use the phytoplankton biomarker (brassicasterol, dinosterol and alkenones) contents as well as the ratios between these biomarkers in three sediment cores from the ECS shelf to reconstruct the spatiotemporal variations of productivity and community of diatoms, dinoflagellates and coccolithophores during the Holocene, respectively. During 9–7 ka, the ECS shelf was characterized by low phytoplankton productivity with low coccolithophore contribution, caused by the oligotrophic condition mainly owing to the restricted Kuroshio Current (KC) intrusion under low sea-level conditions, thus the lack of nutrient input. Phytoplankton productivity generally increased during 7–4.6 ka, in response to the initial intrusion of the Yellow Sea Warm Current (YSWC, a branch of the KC), bringing nutrient from the subsurface KC to the upper layer of the ECS for phytoplankton growth. Phytoplankton productivity continuously increased during 4.6–1 ka, due to an enhanced circulation system (YSWC and Yellow Sea Coastal Current (YSCC)) driven by strong East Asia Winter Monsoon (EAWM). Significantly, high alkenone contents and coccolithophore contribution in the eastern core F11A was associated with its location closer to the warm and saline YSWC, which was suitable for coccolithophore growth. Beyond diagenetic processes which could partly account for higher biomarker contents near core tops, elevated phytoplankton productivity during the last 1 ka might be induced by more nutrient supply from the intensified circulation system driven by enhanced KC and anthropogenic activities. The latter also resulted in high dinoflagellate proportions in all three cores. These temporal and spatial changes of phytoplankton productivity and community structure in the ECS during the Holocene corresponded to different mechanisms by the air-sea interaction, providing insights into distinguishing natural forcing and anthropogenic influences on marine ecology.