Lithology, micropaleontology and isotope record of sediments from the Nordic Seas

On the basis of various lithological, mircopaleontological and isotopic proxy records covering the last 30,000 calendar years (cal kyr) the paleoenvironmental evolution of the deep and surface water circulation in the subarctic Nordic seas was reconstructed for a climate interval characterized by intensive ice-sheet growth and subsequent decay on the surrounding land masses. The data reveal considerable temporal changes in the type of thermohaline circulation. Open-water convection prevailed in the early record, providing moisture for the Fennoscandian-Barents ice sheets to grow until they reached the shelf break at ~26 cal. kyr and started to deliver high amounts of ice-rafted debris (IRD) into the ocean via melting icebergs. Low epibenthic delta18O values and small-sized subpolar foraminifera observed after 26 cal. kyr may implicate that advection of Atlantic water into the Nordic seas occurred at the subsurface until 15 cal. kyr. Although modern-like surface and deep-water conditions first developed at ~13.5 cal. kyr, thermohaline circulation remained unstable, switching between a subsurface and surface advection of Atlantic water until 10 cal. kyr when IRD deposition and major input of meltwater ceased. During this time, two depletions in epibenthic delta13C are recognized just before and after the Younger Dryas indicating a notable reduction in convectional processes. Despite an intermittent cooling at ~8 cal. kyr, warmest surface conditions existed in the central Nordic seas between 10 and 6 cal. kyr. However, already after 7 cal. kyr the present day situation gradually evolved, verified by a strong water mass exchange with the Arctic Ocean and an intensifying deep convection as well as surface temperature decrease in the central Nordic seas. This process led to the development of the modern distribution of water masses and associated oceanographic fronts after 5 cal. kyr and, eventually, to today's steep east-west surface temperature gradient. The time discrepancy between intensive vertical convection after 5 cal. kyr but warmest surface temperatures already between 10 and 6 cal. kyr strongly implicates that widespread postglacial surface warming in the Nordic seas was not directly linked to the rates in deep-water formation.本研究基于覆盖过去3万年（cal kyr）的各类岩性记录（lithological records）、微古生物学记录（micropaleontological records）及同位素代用指标记录（isotopic proxy records），针对周边陆块经历强烈冰盖生长与后续消融的气候时段，重建了亚北极挪威海（subarctic Nordic seas）深水与表层水环流的古环境演化过程。数据揭示了温盐环流（thermohaline circulation）类型存在显著的时间变化。早期记录中以开阔水体对流（open-water convection）为主，为芬诺斯坎底亚-巴伦支冰盖（Fennoscandian-Barents ice sheets）的生长提供水汽，直至该冰盖在约2.6万cal kyr时扩张至陆架坡折带（shelf break），并开始通过消融冰山向海洋输入大量冰筏碎屑（ice-rafted debris, IRD）。2.6万cal kyr之后观测到的低底栖δ¹⁸O（epibenthic delta18O）值与小型亚极有孔虫（subpolar foraminifera），或表明大西洋水平流（advection of Atlantic water）以次表层（subsurface）的方式进入挪威海，直至1.5万cal kyr。尽管现代型表层与深水环境最早于约1.35万cal kyr形成，但温盐环流仍不稳定，在大西洋水次表层平流与表层平流之间切换，直至1.0万cal kyr时冰筏碎屑沉积与融水大规模输入均停止。在此期间，新仙女木事件（Younger Dryas）前后各出现一次底栖δ¹³C（epibenthic delta13C）低值，指示对流过程（convectional processes）出现显著减弱。尽管约0.8万cal kyr存在一次间歇性降温（intermittent cooling），但1.0万至0.6万cal kyr期间挪威海中部存在最暖的表层环境。然而，早在0.7万cal kyr之后，现代海洋格局逐渐形成：这一点可通过与北冰洋的强烈水团交换（water mass exchange）、不断增强的深层对流（deep convection），以及挪威海中部表层温度下降得到验证。该过程在0.5万cal kyr之后造就了现代水团分布与相关海洋锋面（oceanographic fronts），并最终形成了现今显著的东西向表层温度梯度（east-west surface temperature gradient）。0.5万cal kyr之后出现强烈垂直对流，而最暖表层温度却出现于1.0万至0.6万cal kyr，二者的时间错位强烈表明，挪威海冰川后期广泛的表层变暖，与深水形成速率（deep-water formation）并无直接关联。