Late Pleistocene to Holocene changes in upper-ocean stratification and its impact on marine productivity, sea surface temperatures, and salinity in the subarctic Northwest Pacific
The modern subarctic North Pacific realm is characterized by a steep vertical gradient in salinity, the permanent halocline. The halocline not only hampers the exchange of gas and heat between the deep ocean and the atmosphere, it also restricts the supply of nutrients from the subsurface into the photic zone, thereby limiting marine productivity. However, for the subarctic North Pacific and its marginal seas it remains unclear whether low glacial productivity was controlled by upper-ocean stratification or by sea-ice dynamics. Recent proxy and modelling studies indicate that the halocline weakened during the last glacial termination. This thesis is based on piston cores recovered from intermediate water levels in the hitherto only poorly studied western Bering Sea, the continental slope off eastern Kamchatka, and the southern Okhotsk Sea. Age models rely on a combination of benthic oxygen isotope stratigraphy, magnetostratigraphy, radiocarbon dating, and intercore correlations via high-resolution core logging data. Millennial-scale changes in marine productivity and terrigenous matter supply were reconstructed over the past 180 kyr applying a geochemical multi-proxy approach in combination with XRF logging data. Special emphasis is given to the last glacial termination, for which high-resolution sea surface temperature (SST), as well as subsurface temperature and salinity-approximating records were produced. These records stem from alkenone-thermometry, and from the combined measurement of stable oxygen isotope and Mg/Ca ratios of the planktonic foraminifera Neogloboquadrina pachyderma (sin.). Direct comparison between both temperature reconstructions allowed to infer oceanographic changes in the mixed layer. Deglacial changes in the ventilation of intermediate and deep waters are inferred from marine radiocarbon ages and stable carbon isotope records. Results point towards low marine productivity but high terrigenous inputs during most of the last glacial-interglacial cycle and Marine Isotope Stage (MIS) 6. Increases in marine productivity occurred during warm stages of MIS5, interglacials, and the last glacial termination. These changes are supposedly controlled by insolation and sea-level changes, thereby influencing the strength of atmospheric pressure systems, seasonal contrasts, and sea-ice rafting. Sea-ice dynamics are thought to drive changes in surface productivity, terrigenous inputs, and upper-ocean stratification. Abrupt environmental changes recorded during the last 180 kyr apply to the deglacial situation and are potentially related to North Atlantic Dansgaard-Oeschger events. During the last glacial termination SST records matched the variability in sea-ice extent, thereby resembling climate oscillations registered in Greenland ice cores. This variability is explained by perturbations of the Atlantic Meridional Overturning Circulation and suggests a quasi-synchronous atmospheric coupling between the North Pacific and the North Atlantic. Subsurface conditions differed on a regional scale, thereby reflecting different sea-ice influence. Deglacial thermocline changes are proposed with reduced thermal stratification of the upper water column during the Heinrich Stadial 1 (H1) and the Younger Dryas cold phases, but strengthened thermal stratification during the Bølling-Allerød warm phase. Moreover, enhanced intermediate water formation in the Bering and Okhotsk seas and according shallow overturning in the subarctic northwest Pacific occurred during H1, supporting the notion of fast atmospheric interactions between the North Pacific and the North Atlantic which are held responsible for this situation. Results for the Holocene surface and subsurface development do not support the hypothesis of the Atlantic–Pacific Seesaw, but imply that modern upper-ocean conditions were established in the early Holocene.
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