Reconstruction of past changes in ocean salinity - a compound specific stable hydrogen isotope approach
Kasper, S. (2015). Reconstruction of past changes in ocean salinity - a compound specific stable hydrogen isotope approach. PhD Thesis. Universiteit Utrecht (UU): Utrecht. ISBN 978-94-6203-839-4. 140 pp. hdl.handle.net/1874/313601
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| Abstract |
The extent of the general warming related to increasing anthropogenic CO2 emission andits implications for the global climate system are currently under heavy debate. In particular theextrapolation of long term climatic trends relies on complex climate models for the interactionbetween the atmosphere, the oceans and the land surface. In turn these models require validationbased on continuous time series of observational data. However, instrumental based data recordsonly extend as far back as approximately 150 years and thus make the validation of long-termmodelling experiments difficult. To obtain information on past climate beyond the instrumentalperiod, so-called paleoceanographic proxies are used. Two of the most targeted paleoceanographicparameters are the sea surface temperature (SST) and sea surface salinity, which determine waterdensity. While past ocean temperatures can be reconstructed relatively accurately from variousindependent methods, the accurate reconstruction of past ocean salinity has proven to bemore difficult. A promising new method for the reconstruction of past ocean salinity has beensuggested to come from the hydrogen isotope composition of long chain alkenones (dDalkenone)derived from marine haptophyte algae. Culture studies have shown a strong correlation betweenhydrogen isotope fractionation and salinity, with decreasing fractionation with increasing salinity.In order to test the applicability of the dDalkenone as a salinity proxy, this thesis presents studies onsedimentary records obtained from various open ocean settings including the greater AgulhasSystem south of the African continent, the Eastern Tropical Atlantic and the Atlantic sector ofthe Southern Ocean, as well as coastal ocean margin settings in the Mozambique Channel andoffshore Southeast Australia.The dDalkenone from two sediment records located in the Agulhas Leakage area, covering thelast two glacial terminations, showed substantial hydrogen isotopic shifts from more D-enrichedalkenones during the glacial towards relatively more D-depleted alkenones during the interglacials.This pattern was in good agreement with planktonic foraminiferal based sea water oxygen isotope(d18Osw) records, suggesting a glacial-interglacial freshening in the Agulhas leakage area. Thistrend could be explained by the generally weaker Agulhas Leakage during glacial periods as aresponse to changing wind field stress, the expansion of sea ice and the northward displacementof the subtropical fronts. As a result, the transport of Indian Ocean water into the Atlantic waslikely less efficient and a potentially increased back transport of the Indian Ocean water occurredvia the Agulhas Return Current, possibly leading to more saline conditions during glacials andsubsequent freshening during the glacial terminations.Application of the dDalkenone salinity proxy in the upstream region of the Agulhas Currentprovided evidence that upper water mass salinities in the Agulhas leakage signal might have beencontrolled by upstream dynamics in the southern Agulhas Current without a significant change inthe actual Agulhas leakage into the South Atlantic. Instead, more saline southern Agulhas Currentwaters were potentially propagated to the Agulhas Leakage area during glacial termination periodsimplying that periods of higher salinity in the source region did not necessarily entail a greaterfraction of Indian Ocean water entering the South Atlantic Ocean, as initially inferred from thesalinity proxy records in the Agulhas Leakage area.Summary128A comparison of the hydrogen isotope fractionation factor aalkenone-sw with d18Oswreconstructions in the eastern tropical Atlantic Ocean showed similar patterns, suggesting bothproxy records reflected salinity. Differences were observed for the period between marine isotopestage (MIS) 5 - MIS2 where d18Osw reconstructions showed only little variation, while aalkenone-swindicated more pronounced variability. This is potentially related to variability in growth rate of thealkenone producers. However, a comparison of the dDalkenone record with d18Osw reconstructionsfrom the western subtropical Atlantic showed a fairly good agreement during this time period,suggesting that changes in salinity occurred basin-wide in the tropical Atlantic. The relativelygood agreement of the dDalkenone with the carbon isotope record of benthic foraminifera suggest aconnection between AMOC strength variability and relative salinity shifts during MIS5 and MIS6as well as during MIS3 and MIS4 at the core site.The dDalkenone was used to trace allochthonous input of alkenones in the Atlantic Sector of theSouthern Ocean. Here, SSTs reconstructed using the distribution of alkenones indicate a relativelywarm glacial, in contrast to distinctively cold glacial SST conditions indicated temperature recordsbased upon planktonic foraminiferal d18O values and distribution of Thaumarchaeotal lipids. Theobserved shift towards more negative values in dDalkenone during the glacial – interglacial transitionwas only slightly higher than expected based on global ice volume changes. This suggests thatindeed the alkenones might have been allochtonous and synthesized in warmer water masses withsimilar or slightly higher salinity during the glacial and transported to the core site.A 39 kyr record of dDalkenone values obtained from the Eastern African continental shelfin the Mozambique Channel showed, in contrast to open ocean records, no clear glacial –interglacial trend. In contrast to present day, the core site was in closer proximity to the continentduring glacial sea level low stand, resulting in a much larger effect of freshwater run off. Theseconditions probably led to relatively low salinity and therefore an increased isotopic fractionationof hydrogen during alkenone biosynthesis, resulting in the apparent lack of glacial-interglacialtrend in the dDalkenone record. On top of that a positive correlation between dDalkenone and valuesfor the branched isoprenoid tetraether (BIT) index, a proxy for the input of soil organic matter, isobserved during the glacial period. This possibly indicated an increasing contribution to alkenoneproduction by coastal haptophyte species, which is supported by elevated ratios of C37/C38alkenones.In contrast, a coastal marine 135 kyr dDalkenone record off the continental shelf of SouthAustralia in front of the River Murray did show a strong glacial – interglacial pattern, in goodagreement with a planktonic foraminiferal d18O record. Despite increased terrestrial influence, asinferred from elevated BIT values, during sea level low stands via the Murray river, records ofalkenone accumulation rate and the alkenone C37/C38 ratio suggested that species compositionor growth rate changes in the haptophyte producers did not substantially affect the dDalkenonevariability in this record. This suggests that dDalkenone record at this core site was influenced byfreshwater runoff to a lesser extent than that in the Mozambique Channel. In general, the glacialperiods (i.e. late MIS6, MIS4 and MIS2) were marked by more D-enriched alkenones compared tointerglacial periods, indicating a freshening during glacial terminations. This input of low salinitywater is likely derived from the southwards flowing Leeuwin Current, which was elevated duringdeglaciations and interglacials.Summary129In summary, results described in this thesis have shown that the hydrogen isotope fractionationas reflected in long chain alkenones is a promising proxy for assessing changes in past ocean salinity.Multi-millennial sediment records of dDalkenone generally agreed well with planktonic foraminiferad18O records from open ocean and near coastal settings. Estimation of absolute salinity shifts,based on culture derived salinity aalkenone-sw relationships also corresponded reasonably well withestimations based on planktonic foraminifera d18Osw reconstructions. However, using aalkenone-swfor calculating absolute salinities requires an estimation of the hydrogen isotope composition ofthe past ocean, which results in larger uncertainties. This problem may be resolved in the futureby using multiple relationships of a for different compounds, e.g. sterols. |
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