Use of algal biomarkers from marine sediments for reconstructing Antarctic environmental change in the Holocene

Ashley, Kate (2020). Use of algal biomarkers from marine sediments for reconstructing Antarctic environmental change in the Holocene. University of Birmingham. Ph.D.

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Abstract

Antarctic coastal zones play a key role in regulating global climate, through sea level change, ocean circulation and the global carbon cycle. Observations over recent decades have documented changes such as thinning of ice sheets, freshening of the surrounding ocean and changes in sea ice extent. However, these records are only short and sparse leaving many questions yet to be answered about the drivers of such changes and environmental implications. Palaeoenvironmental records provide the potential to reconstruct such changes over the past and extend observations back through time. This requires a robust understanding of how different proxies work and thus what signal they are recording. This thesis aims to assess the use of two biomarker proxies for reconstructing past environmental conditions in the Antarctic coastal zone, namely compound-specific carbon isotopes of fatty acids and the TEX\(_{86} ^L\) sea surface temperature proxy, as well as applying these and other proxies to reconstruct past environmental conditions. Two marine sediment cores recovered from off the coast of Adélie Land, East Antarctica, are used for this work: a short core spanning roughly the past four centuries, and another core spanning the Holocene.

Changes in concentrations and carbon isotopes of two fatty acids in sediment samples are analysed in the short core and compared with down-core changes in other proxy data. Different possible drivers of fatty acid δ13C are explored, of which primary productivity changes appear to be the most likely driver. The C24 fatty acid δ13C is suggested to be predominantly reflecting productivity in open water environments, while C18 fatty acid δ13C is reflecting productivity in the marginal ice zone. Fatty acid δ13C is applied to the Holocene sediment core to reconstruct primary productivity changes. These data suggest that primary productivity was spatially variable over the Holocene. Open water productivity appears to be highest and most stable during the middle and late Holocene periods, after local ice sheets have retreated. Despite a rapid and widespread increase in sea ice concentration in the Late Holocene, open water productivity remains high, possibly reflecting conditions within the Mertz Glacier Polynya, up-wind from the core site. In contrast, ice edge productivity reaches the lowest levels, perhaps due to a shorter ice-free season leading to a lack of stratified, nutrient-rich water.

A high-resolution record of TEX86 L from off the coast of Adélie Land, East Antarctica is presented spanning the last four centuries. The TEX86 L record, however, shows no correlation with air temperature in this region over the period of temporal overlap. This, in addition to a consistent over-prediction of temperatures compared to known sea surface temperatures (SST) in the region, and the deviation of glycerol dialkyl glycerol tetraether (GDGT) distributions from the global core top dataset, suggest that TEX86 L is not recording SST. A correlation between TEX86 L and Sand% suggests that TEX86 L is being driven by changes in the strength of the westward coastal currents. We hypothesise that stronger currents are leading to greater advection of GDGTs from further east, where modified Circumpolar Deep Water (MCDW) upwells. MCDW is known to have a greater diversity of archaeal communities and thus, greater advection may lead to greater inputs of GDGTs produced by different archaea, for which the relationship between GDGT cyclisation and temperature may differ.

A range of proxies are applied to both sediment cores to reconstruct past changes in different environmental conditions. In the Holocene sediment core, fatty acid hydrogen isotopes indicate a peak in meltwater input during the mid-Holocene which coincides with a widespread increase in sea-ice around the coast, persisting towards the modern day. This is hypothesised to reflect the development of a large ice-shelf cavity in the Ross Sea which fundamentally alters ice-ocean interactions and surface water properties. These surface waters are transported west, enhancing sea ice formation around the coast. This provides a new insight into ice shelf-ocean-sea ice interactions with implications for the incorporation of ice shelves into climate models. Proxy data from the short-core provides a high-resolution insight into the relationship between different environmental factors over the past four centuries. These data highlight the key role of sea ice in driving changes in ice-edge primary productivity, inputs of glacial meltwater and strength of the westward coastal current. Iceedge primary productivity broadly increases when both fast and pack ice are low. Productivity in more open waters, however, appears to be less sensitive to changes in sea ice. Inputs of glacial meltwater also appear to be controlled by sea ice concentrations, perhaps due to isotopically-depleted meltwater being locked up in sea ice and released when extensive melting of sea ice occurs. The strength of the westward current appears to be affected by the concentration of fast ice attached to the coast, due to its effect on wind stress. Comparison with instrumental data is difficult due to the short-term nature of the overlapping datasets. However, there is some indication that wind direction may play a key role in determining sea ice concentrations in this location which, in turn, affects many other environmental conditions offshore Adélie Land.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):