Oceanographic and biogeochemical change in the high-latitude North Atlantic during marine isotope stage 11 and the Common Era

Abstract

The high-latitude North Atlantic is critically involved in Earth’s climate system. In polar and subpolar regions, the formation of deep-water masses regulates circulation throughout the global ocean, thereby also modulating heat, nutrient and phytoplankton dynamics. Several lines of evidence suggest that North Atlantic circulation has weakened over the last century, which is likely due to increased high-latitude warming and freshening. While climate models predict a continued weakening of Atlantic circulation over the next century, longer-term variations are uncertain. Further, the biological and biogeochemical consequences of such physical-oceanographic changes are not well constrained due to additional potential drivers of high-latitude primary productivity, such as variations in light attenuation and possible trophic cascades induced by industrial fishing.

The marine geological record provides a useful framework for assessing the consequences of relatively intense warming on oceanic circulation. In this regard, the marine isotope stage (MIS) 11 interglacial period, from 424 to 374 ka, has been proposed as a paleo-analog to Earth’s contemporary and near-future climate due to its similarities in orbital geometry and preindustrial atmospheric chemistry. Interestingly, paleoceanographic studies suggest that a vigorous Atlantic circulation was maintained throughout MIS 11, despite prolonged freshwater input reconstructed in the high-latitude North Atlantic. To understand the spatiotemporal behavior of deep-water formation during this interval, I conducted geochemical reconstructions of upper-ocean structure and deep-ocean ventilation in the polar Nordic Seas and subpolar Iceland Basin – two sites of contemporary convection. My data suggest that extended freshwater input to the Nordic Seas triggered deeper mixing during the climatic optimum, thereby possibly preconditioning the polar region for convection. Deep-water formation in the Nordic Seas could have likely contributed to enhanced northern-hemisphere warming and extended interglacial conditions during MIS 11. This finding may have relevance for the long-term fate of oceanic circulation under an increasingly warmer and fresher North Atlantic.

Disentangling the impacts of ongoing circulation changes on nutrient dynamics and productivity is complicated due to the existence of several plausible physical and biological drivers. To understand such changes along the Labrador Shelf, located in the western subpolar North Atlantic, I analyzed nitrogen isotope data derived from a six-hundred-year-old crustose coralline alga in the context of multiple other high-resolution paleoenvironmental datasets. I found that Atlantic inflow indeed plays a dominant role in supplying nitrate, a key nutrient, to the shelf region, which had historically been linked to internal multidecadal variability. However, my analysis suggests that an anomalously prolonged reduction of nitrate inflow has occurred since ~1870, which I hypothesize is linked to the contemporary weakening of larger-scale circulation in the North Atlantic. Along west Greenland, additional algal geochemical data reveal associations between industrial fishing, productivity and nitrate utilization over the last century. Specifically, these data suggest that the removal of North Atlantic cod likely triggered a trophic cascade along the west coast, resulting in increased phytoplankton abundance at ~1960. As phytoplankton continue to play central roles in other socioeconomically important fisheries, ecosystem dynamics and carbon cycling, constraining their drivers is crucial for better understanding a wide suite of environmental phenomena.