Sea surface density gradients in the Nordic Seas during the Holocene as revealed by paired microfossil and isotope proxies. (18th March 2016) - Van Nieuwenhove, Nicolas
Abstract: We attempt to assess the Holocene surface‐subsurface seawater density gradient on millennial time scale based on the reconstruction of potential density ( σθ ) by combining data from dinoflagellate cyst assemblages and planktic foraminiferal ( Neogloboquadrina pachyderma (s)) stable oxygen isotopes ( δ 18 Oc ). Following several calibration exercises, the likeliness of favorable seasonal preconditioning to open ocean convection is evaluated. The reconstructed σθ values reveal unfavorable conditions for vertical convection in the western Nordic Seas prior to ~7–6.5 ka B.P., with a westward increase and persistence of surface water buoyancy. Active overturning became more likely after 6.5 ka B.P. as suggested by a reduced and recurrently inverted vertical σθ gradient, while intermittent eastward spreading of lower density surface waters continued to modulate the area of potential overturning. Despite some reservation regarding the accuracy of the σθ values reconstructed, the documentation of relative changes of σθ gradients through time and space is suggested as a helpful tool for the appraisal of past overturning likeliness. Key Points: Density gradients are reconstructed by combining plankton and isotope data Freshwater input stratified the upper ocean well into the Holocene Active overturning became likely after 6.5 ka B.P. in the western Nordic Seas