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HB-1-10

Modeling the effect of meltwater on the habitat of Arctic planktonic foraminifera

PhD student :Kerstin Kretschmer
Project supervisors :M. Kucera, M.Schulz (Germany)
C. Hillaire-Marcel (Canada)
Key hypothesis
Existing meltwater reconstructions based on Neogloboquadrina pachyderma systematically underestimated the true anomaly and an ecosystem modeling approach allows for correcting the offset.
Project description
Fossil shells of planktonic foraminifera are the prime source of information on surface water conditions in high latitudes. Isotopic and trace-element signatures locked in the calcite of their shells have been used to quantify past changes in surface ocean chemistry, stratification and hydrology. In particular, in the Arctic and North Atlantic, oxygen isotopes in planktonic foraminifera shells helped identify, trace and characterize large-scale meltwater events and sea-ice formation patterns. Quantitative reconstructions of surface water properties from planktonic foraminifera critically depend on the exact origin in the water column and in time of the signal preserved in the sediment. This signal is biased towards the time and depth of maximum production and it may entirely exclude information from habitat states that are not conducive for the survival of foraminifera species or for their calcification, such as hyposaline meltwater lenses that may develop under highly stratified conditions. Whereas the season of production of arctic planktonic foraminifera can be reasonably approximated to occur during the productive summer months, their vertical habitat and calcification pattern is much more complex. Specifically, it has been demonstrated that the main polar species Neogloboquadrina pachyderma calcifies preferentially below the near-surface pycnocline in the North Atlantic and Arctic. Salinity reconstructions using this species often assume that calcification occurs close to the surface. The vertical offset between assumed and actual calcification depth is particularly relevant for the reconstruction of past meltwater influx from ice sheets. It is likely, that the oxygen-isotope composition of N. pachyderma shells, which formed below a meltwater layer, will systematically underestimate a true salinity anomaly. Recently, major progress has been achieved in modeling the distribution of planktonic foraminifera globally at a species level. These models are based on marine ecosystem models, forced by changes in physical boundary conditions. They provide the best available perspective to facilitate robust interpretation of the magnitude of past meltwater events, surface ocean properties and stratification, as well as an assessment of the behavior of carbonate production by planktonic foraminifera in the Arctic under a variety of global change scenarios. This new class of models has already been successfully used to quantify changes in species distribution between the Last Glacial Maximum and today. A major limitation of all available models is that the vertical distribution of planktonic foraminifera in the water column is currently not resolved, that is, all foraminifera are assumed to occur in the surface mixed layer. Model resolution in the vertical dimension is not trivial, but can benefit from rich data on vertical occurrences and temporal fluxes of the key species throughout the Arctic realm. A combination of observations from across the Arctic is justified for N. pachyderma, since molecular genetic studies have revealed that its North Atlantic and Arctic populations are genetically homogenous and represented by a single interbreeding population.
The PhD project will test this hypothesis by assessing the potential bias of foraminifera-based oxygen-isotope records due to meltwater-induced variations in the depth habitat and seasonal succession of planktonic foraminifera. Towards this goal we propose to combine the foraminifera ecosystem model of Fraile et al. (2008) with a depth-habitat module. An additional module will be developed to calculate species- specific oxygen-isotope composition of the modeled foraminiferal shells. The sensitivity to salinity will be parameterized on the basis of manipulative experiments carried out in the Arctic on board a research vessel. The geochemical-foraminiferal model will be used to assess the impact of meltwater injections into the Arctic and North Atlantic Ocean during Heinrich Events, the Younger Dryas and the 8.2-kyr event on foraminiferal oxygen-isotope records. By comparing the model output with paleoceanographic reconstructions (existing and through HB-8 and 9) and meltwater histories (HB-11, CA-14) it will be possible to correct for the habitat effect on oxygen-isotope composition and to obtain more robust estimates of past meltwater fluxes.