On the stability of various highly branched isoprenoid (HBI) lipids in stored sediments and sediment extracts

publisher: Elsevier articletitle: On the stability of various highly branched isoprenoid (HBI) lipids in stored sediments and sediment extracts journaltitle: Organic Geochemistry articlelink: http://dx.doi.org/10.1016/j.orggeochem.2016.04.010 content_type: article copyright: © 2016 Elsevier Ltd. All rights reserved

Antiphased dust deposition and productivity in the Antarctic Zone over 1.5 million years

The Southern Ocean paleoceanography provides key insights into how iron fertilization and oceanic productivity developed through Pleistocene ice-ages and their role in influencing the carbon cycle. We report a high-resolution record of dust deposition and ocean productivity for the Antarctic Zone, close to the main dust source, Patagonia. Our deep-ocean records cover the last 1.5 Ma, thus doubling that from Antarctic ice-cores. We find a 5 to 15-fold increase in dust deposition during glacials and a 2 to 5-fold increase in biogenic silica deposition, reflecting higher ocean productivity during interglacials. This antiphasing persisted throughout the last 25 glacial cycles. Dust deposition became more pronounced across the Mid-Pleistocene Transition (MPT) in the Southern Hemisphere, with an abrupt shift suggesting more severe glaciations since ~0.9 Ma. Productivity was intermediate pre-MPT, lowest during the MPT and highest since 0.4 Ma. Generally, glacials experienced extended sea-ice cover, reduced bottom-water export and Weddell Gyre dynamics, which helped lower atmospheric CO2 levels

Identification of C 25 highly branched isoprenoid (HBI) alkenes in diatoms of the genus Rhizosolenia in polar and sub-polar marine phytoplankton.

We report the identification of a range of C25 highly branched isoprenoid (HBI) alkenes and certain sterols in filtered phytoplankton samples obtained from western Svalbard (Arctic) and near South Georgia (South Atlantic, sub-Antarctic) in 2016 and 2014, respectively. The C25 HBIs contained 3–5 double bonds and had structures identified previously from analysis of laboratory diatom cultures. The same HBIs were also identified in individual diatom taxa isolated from the mixed assemblages and with reasonably similar distributions. Thus, C25 HBIs were identified in Rhizosolenia setigera isolated from western Svalbard near-surface waters, while the same HBIs were also found in R. polydactyla f. polydactyla and R. hebetata f. semispina picked from seawater collected from a site in the South Atlantic. The main sterol composition was slightly different between the two locations, with cholesta-5,24-dien-3β-ol (desmosterol) identified as one of the major components in the sample from West Svalbard, consistent with the diatom assemblage being dominated by R. setigera. In contrast, the major sterol in the South Atlantic sample was cholesta-5,22-dien-3β-ol (22-dehydrocholesterol), likely reflecting the relatively high proportion of the genus Pseudo-nitzschia. For both locations, the suite of HBIs included a tri-unsaturated isomer (HBI III; 6Z-2,6,10,14-tetramethyl-9-(3'-methylpent-4-enylidene)-pentadec-6-ene), proposed in previous studies as a potential proxy measure of pelagic sea ice-edge conditions, and thus, a counterpart to the mono- and di-unsaturated HBIs IP25 and IPSO25, which have been used as seasonal sea ice proxies in the Arctic and Antarctic, respectively. HBI III has been reported previously in sediments from West Svalbard and we report here its occurrence in a small number of surface sediments from the South Atlantic. For both regions, HBI III was present as one of the major HBIs in sediments, which contrasts the HBI distributions in the filtered phytoplankton samples, where HBIs with four and five double bonds were the major components. Differences in HBI distributions between phytoplankton and sediment samples may potentially be due to the presence of other (unanalysed) diatoms in the filtered water samples, seasonal/annual variability in the production of HBIs by a range of diatoms, differential degradation of HBIs between sources and sediments, or a combination of these. Interestingly, we did not detect any C30 HBIs in the water samples, picked cells or sediments from either location, despite earlier reports of these lipids in laboratory cultures of R. setigera. This study represents the first source identification of certain C25 HBI lipids under in situ pelagic conditions

Antiphased dust deposition and productivity in the Antarctic Zone over 1.5 million years

The Southern Ocean paleoceanography provides key insights into how iron fertilization and oceanic productivity developed through Pleistocene ice-ages and their role in influencing the carbon cycle. We report a high-resolution record of dust deposition and ocean productivity for the Antarctic Zone, close to the main dust source, Patagonia. Our deep-ocean records cover the last 1.5 Ma, thus doubling that from Antarctic ice-cores. We find a 5 to 15-fold increase in dust deposition during glacials and a 2 to 5-fold increase in biogenic silica deposition, reflecting higher ocean productivity during interglacials. This antiphasing persisted throughout the last 25 glacial cycles. Dust deposition became more pronounced across the Mid-Pleistocene Transition (MPT) in the Southern Hemisphere, with an abrupt shift suggesting more severe glaciations since ~0.9 Ma. Productivity was intermediate pre-MPT, lowest during the MPT and highest since 0.4 Ma. Generally, glacials experienced extended sea-ice cover, reduced bottom-water export and Weddell Gyre dynamics, which helped lower atmospheric CO2 levels

Late quaternary sea-ice and sedimentary redox conditions in the eastern Bering Sea – Implications for ventilation of the mid-depth North Pacific and an Atlantic-Pacific seesaw mechanism

On glacial-interglacial and millennial timescales, sea ice is an important player in the circulation and primary productivity of high latitude oceans, affecting regional and global biogeochemical cycling. In the modern North Pacific, brine rejection during sea-ice freezing in the Sea of Okhotsk drives the formation of North Pacific Intermediate Water (NPIW) that ventilates the North Pacific Ocean at 300 m to 1000 m water depth. Glacial intervals of the late Quaternary, however, experienced a deepening of glacial NPIW to at least 2000 m, with the strongest ventilation observed during cold stadial conditions of the last deglaciation. However, the origin of the shifts in NPIW ventilation is poorly understood. Numerical simulations suggest an atmospheric teleconnection between the North Atlantic and the North Pacific, in response to a slowdown or shutdown of the Atlantic meridional overturning circulation. This leads to a build-up of salinity in the North Pacific surface ocean, triggering deep ventilation. Alternatively, increased sea-ice formation in the North Pacific and its marginal seas may have caused strengthened overturning in response to enhanced brine rejection. Here we use a multi-proxy approach to explore sea-ice dynamics, sedimentary redox chemistry, and benthic ecology at Integrated Ocean Drilling Program Site U1343 in the eastern Bering Sea across the last 40 ka. Our results suggest that brine rejection from enhanced sea-ice formation during early Heinrich Stadial 1 locally weakened the halocline, aiding in the initiation of deep overturning. Additionally, deglacial sea-ice retreat likely contributed to increased primary productivity and expansion of mid-depth hypoxia at Site U1343 during interstadials, confirming a vital role of sea ice in the deglacial North Pacific carbon cycle

Development of biomarker-based proxies for paleo sea-ice reconstructions

The analysis of the sea-ice diatom biomarker IP25 (a mono-unsaturated Highly Branched Isoprenoid (HBI) alkene) in Arctic marine sediments has previously been shown to provide a useful qualitative proxy measure for the past spring sea-ice occurrence. In the Southern Ocean the occurrence and variable abundance of a structurally similar di-unsaturated HBI (HBI diene II) has previously been proposed as a proxy measure of paleo sea-ice extent. However, the use of such biomarker proxies remains under development. In the current study, a number of additional palaeoceanographic developments of HBIs as sea-ice biomarkers in both polar regions has been undertaken. For the Arctic, an investigation into the combined analysis of IP25 and certain phytoplankton biomarkers has been conducted with the aim of providing more detailed and semi-quantitative descriptions of sea-ice conditions in the Barents Sea. In contrast, analysis of HBIs and other lipids within water column, surface sediment and sea-ice samples has been undertaken to provide further insights into the use of HBIs as proxies for Antarctic sea-ice. Analysis of surface sediments from across the Barents Sea has shown that the relative abundances of IP25 and a tri-unsaturated HBI lipid (HBI triene IIIa) are characteristic of the overlying surface oceanographic conditions, most notably, the location of the seasonal sea-ice edge. A semi-quantitative approach, in the form of the PIP25 index, showed a good positive linear relationship between PIP25 indices and spring sea-ice concentration, with a particularly strong relationship found when using HBI triene IIIa (PIIIaIP25) as the open-water counterpart to IP25. The quality of the linear fits were not especially dependent on the balance factor c, used in the PIP25 calculation, which may have important positive consequences for down-core sea-ice reconstruction, and when making comparisons between outcomes from different Arctic regions or climatic epochs. Further, a lower limit threshold for PIIIaIP25 (0.8) might represent a useful qualitative proxy for the past occurrence of summer sea-ice. The re-evaluation of biomarker data from three dated marine sequences in the Barents Sea suggests that the combined analysis of IP25 and HBI triene IIIa can provide information on temporal variations in the position of the maximum (winter) Arctic sea-ice extent, together with more quantitative sea-ice reconstructions. In the Southern Ocean, the distributions of di- and tri-unsaturated HBIs (HBI diene II and HBI trienes IIIa and IIIb) in surface waters were shown to be extremely sensitive to the local sea-ice conditions, consistent with significant environmental control over their biosynthesis by sea-ice diatoms and open water phytoplankton, respectively. Within the water column, the apparent alteration to HBI and other lipid abundances was evident between the photic and benthic parts of the water column, which, along with additional local factors (e.g. polynya formation), may have important implications for paleo sea-ice reconstructions. The sedimentary occurrence and distribution of HBI diene II (termed here as IPSO25) were consistent with the recent identification of the diatom Berkeleya adeliensis Medlin as a source of IPSO25. The tendency for B. adeliensis to flourish in platelet ice, the formation of which is strongly associated with super-cooled freshwater inflow, means that sedimentary IPSO25 may provide a potentially sensitive proxy indicator of landfast sea-ice influenced by meltwater discharge from nearby glaciers and ice shelves. Re-examination of some previous IPSO25 down-core records supports this suggestion, although further down-core analysis is required to confirm this hypothesis. The similar sedimentary distribution relationship between phytoplankton-derived HBI trienes and IPSO25, further indicates that the former may reflect production of these biomarkers by certain diatoms that flourish within the region of the retreating ice edge; however, the source identification of the HBI trienes is still needed to place this interpretation on a firmer footing

HBI concentrations and classification tree model predictions of sea ice conditions for surface sediments and downcore records in the Barents Sea

We investigated the potential for classification tree (CT) models to provide a further approach to paleo Arctic sea ice reconstruction through analysis of a suite of six highly branched isoprenoid (HBI) biomarkers in 198 surface sediments from the Barents Sea. The four CT models representing modern sea ice conditions were then applied to four downcore records within the study area (cores BASICC 1, 8, 43, and core MSM5/5-712-1) in order to reconstruct sea ice conditions over the last 300 years. The current dataset includes the absolute HBI concentrations in all sediment samples (ng/g dry sed.), as well as CT model outcomes for all samples, which were classified as having experienced marginal, intermediate, or extensive overlying sea ice cover (further details are available in the manuscript associated with these data)