Subsurface heat transport in the North Atlantic during the Middle and Late Pleistocene

Abstract

Due to the large thermal inertia of water, the oceans are Earth’s largest heat reservoir and the primary regulator of global climate on long timescales. The ocean absorbs, stores and distributes heat through the planet, and shifting ocean circulation patterns can disrupt established climate regimes. Comparably, the cryosphere modulates the Earth’s climate because ice sheets reflect radiation from the Sun and serve as a reservoir of freshwater, thereby modulating sea level, which affects the large human populations that inhabit coastal zones. Thus, ocean and cryosphere are two significant agents of Earth’s climate system and tightly connected by feedbacks on millennial and orbital timescales. Understanding the internal mechanisms that control such feedbacks is crucial for refining numerical and conceptual models and aid public policy decisions to deal with the repercussions of anthropogenic climate change. However, scientific knowledge about long-timescale ocean-cryosphere interactions is hampered by the geographically sparse and relative short length of observational records. The reconstruction of climate patterns on longer timescales requires the production and interpretation of proxy records and are the scope of this thesis. The close proximity between ice sheets and areas of deep-water formation makes the North Atlantic one of the most climate sensitive areas on the planet. There, the transition zone between the subtropical and subpolar gyres is susceptible to latitudinal shifts of oceanic fronts and was reached by icebergs laden with debris during periods of ice-sheet destabilization since the beginning of the Pleistocene. Additionally, subsurface density layers that connect both gyres at depth are the pathway through which low-latitude heat may reach high latitudes and either feed moisture or accelerate the demise of ice sheets. To assess the relationship between the ocean heat transport and the cryosphere over millennial and orbital timescales, two sets of proxy records were produced from deep-sea marine sediment cores retrieved from the transition zone between both gyres in the North Atlantic. The first set of records allowed for the reconstruction of subsurface temperatures and salinity during the Last Glacial Maximum and last glacial termination, a period of ice-sheet demise. During this time interval, the climate of the northern hemisphere went through several abrupt shifts accompanied by equally abrupt changes in oceanic circulation, that resulted in a sea-level rise of ca. 120 m above the glacial baseline. A compilation of new and published proxy records showed that subsurface warming preceded the deposition of debris transported by icebergs, implicating that northward heat transport triggered or accelerated the demise of glacial ice sheets. A second set of proxy records produced from materials of the same location was used to reconstruct subsurface temperatures at mid-latitudes and oceanic circulation patterns between mid and high latitudes during the Mid-Pleistocene Transition. This interval was characterized by the transition from 40-kyr to 100-kyr glacial-interglacial cyclicity of the Late Pleistocene and was period of significant ice-sheet enlargement. The produced data suggested a prominent role for low-latitude forcing in driving the northward transport of heat when cold surface temperatures and low atmospheric CO2 would have starved ice sheets of moisture and hampered their growth. Additionally, modern-like gradients between mid- and high-latitude oxygen isotope records from the surface, intermediate and deep ocean, indicate the establishment of the Late Pleistocene ocean circulation during this second interval. In ensemble, the data produced within the framework of this thesis were used to suggest a new mechanism of northward heat transport that is closely coupled to the fate of northern hemisphere ice sheets during the Middle and Late Pleistocene. Furthermore, the new data suggest that the causal relationship between northward heat transport and deep-water formation may need to be revaluated to better constrain climate and oceanic circulation in future high-CO2 model scenarios.