No abstract

Freshwater perturbations are often thought to be associated with abrupt climate changes during the last deglaciation, while many uncertainties remain regarding the exact timing, pathway, mechanism, and influence of meltwater release. Here, we present very well-dated and high-resolution records from the eastern Labrador Sea representing the last 19.000 years, which demonstrate abrupt changes in sea surface characteristics. Four millennial-scale meltwater events have been identified between the last 14.000 and 8.200 years based on independent biomarker proxies and X-ray fluorescence scanning data. These events are characterized by increased sea ice formation and decreased sea surface temperatures which might have occurred within a few decades. We propose these abrupt changes were triggered by meltwater pulsing into the Labrador Sea periodically, resulting from collapse of the Laurentide-Greenland Ice Sheets caused by (sub-)surface ocean warming in the Labrador Sea. Our findings provide more precise information about impact of freshwater forcing on abrupt climate changes, which may help to improve simulations for past and future changes in ocean circulation and climate.

<p>The study on the decay of ice sheets in the past provides important insights into the interaction between ice sheet behaviours and ocean characteristics, especially under a sustained warming climate. On the one hand, the ice sheet may affect the ocean environment; on the other hand, changes in sea surface conditions may affect the instability of the ice sheets. However, interactions between ice sheet dynamics and sea surface characteristics are still not fully understood. Thus, studies of carefully selected sediment cores representing both ice-sheet and ocean characteristics can help to better predict changes in ice sheets in the future. Here, we show sedimentary records from the eastern Labrador Sea, proximal to the Laurentide Ice Sheet (LIS) and the Greenland Ice Sheet (GrIS), representing the last 50 ka, i.e., the last glacial-deglacial-Holocene period. Our XRF and biomarker data document the outstanding collapse of the LIS/iceberg discharge during Heinrich Events (i.e., HE5, HE4, HE2, and HE1) and the occurrence of meltwater plumes from the LIS and GrIS during the deglaciation. Such meltwater discharge has caused surface water freshening in the Labrador Sea and, consequently, decreased sea surface temperatures and decreased primary productivity. Enhanced Irminger Current inflow might have triggered the retreat of ice sheets/meltwater discharge, as shown in our planktic foraminifera records. In contrast to dominantly relatively low primary productivity during the glacial period, both higher sea ice algae and phytoplankton production occurred during the Last Glacial Maximum (LGM), probably caused by a polynya in front of the GrIS reaching its maximum extent at that time. During the deglaciation to Holocene time interval, primary productivity shows an increasing trend probably related to decreased meltwater discharge, decreased sea ice extent, and increased insolation.</p> <p> </p>

<p>The last glacial termination is an unstable transition state characterized by abrupt climate changes, while the related physical mechanisms are still not fully understood. Here, we present well-dated high-resolution sedimentary records from the eastern Labrador Sea representing the last 23 ka. Based on our biomarker records, there was seasonal to permanent sea ice cover before 11.7 ka BP. During 11.7 to 8.2 ka BP, ice-free conditions were interrupted by several sea ice expansions, while no sea ice after 8.2 ka BP. Besides Heinrich Event 1, four prominent cold events have been identified during 14 ka to 8.2 ka BP. These abrupt events are marked by increases in sea ice, decreases in sea surface temperature, and weak deep current intensity. We propose that these events were mainly triggered by collapses of the Laurentide Ice Sheet and/or Greenland Ice Sheet, resulting in icebergs/meltwater in pulses into the Labrador Sea. This caused surface freshening, which potentially promoted the stratification of surface water, prevented the northward inflow of Atlantic Water, and limited deep water production in the Nordic Seas, consequently disrupting the climate.</p>