Unit Affiliation: Biology and Paleo Environment, Lamont-Doherty Earth Observatory (LDEO)
Part 1: Polar regions are particularly sensitive to rising temperatures and increasing concentrations of atmospheric greenhouse gases. Currently, the observed warming in the Arctic is almost twice that of the Northern Hemisphere as a whole. A major concern is that further warming of this region could potentially weaken the North Atlantic circulation, which will have a profound influence on climate in Europe and North America. In order to gain a better understanding of the mechanisms that control the rate and magnitude of future climate change on the Northern Hemisphere, we need to acquire robust data from past climate events from this region that can be compared to current observations and help with future predictions. Much of our knowledge of the Earth’s past climate is deduced from chemical analysis of fossil foraminifera shells recovered from drill cores. Foraminifera, which are ubiquitous in the World’s Oceans, are small organisms that form a sand-grain-sized calcium carbonate (calcite) shell. At the end of their life cycle, the empty shell is deposited on the seafloor where it can be preserved for tens of millions of years. While the approach to reconstruct past climate events from fossil foraminifera shells, which was developed in the 1950s and continuously improved over many decades, works well in low- and mid-latitude oceans, deducing accurate past temperatures from foraminifera shells from the Polar North Atlantic remains challenging. Pilot data from fossil foraminifera shells (species Neogloboquadrina pachyderma) from the Polar North Atlantic acquired for this study reveal that the so-called ‘lamellar calcite’, which is the early generation of shell calcite that forms in the upper water column and provides a ‘near-surface’ signal, is often partially or largely dissolved. Thus, the remaining shell comprises mostly the later-formed outer ‘crust calcite’, which records environmental conditions from much greater (typically colder) water depths. As affected shells look unsuspicious – the loss of the lamellar calcite is not visible from the outside when inspected by microscope – shells that have lost their lamellar calcite have likely been selected for analysis in many prior studies. We hypothesize that temperature reconstructions from shells with dissolved lamellar calcite are biased towards colder temperatures. Fortunately, shells with well-preserved lamellar calcite are typically co-occurring with those featuring poor preservation in the same core samples and can thus be selected for analysis. This study aims to carefully select shells with intact lamellar calcite (preserved near-surface signal), and the ratio of Magnesium/Calcium of the lamellar calcite as a chemical recorder of past temperatures will be measured. This is a significant deviation from conventional analytical approaches that average the composition of the whole shell with an unknown proportion of lamellar calcite (warm near-surface signal) and crust calcite (cold deep-water signal). The analytical approach, which will be developed within the framework of this study, has the potential to significantly improve the veracity of paleorecords from the Polar North Atlantic, which is essential to gauge the response of this sensitive region to future greenhouse gas forcing.
Part 2: Climate change in the polar North Atlantic occurs at a much higher rate than most other areas of the World’s Oceans. Therefore, accurate records of past climate variability from this area are essential to improve forecasts of how this sensitive region will react in future scenarios with enhanced greenhouse gas forcing. However, even after decades of research, paleoclimate records from the polar North Atlantic leave many questions unanswered and often do not align with results from model simulations.
The vast majority of sea surface temperature records, reflecting past climate variability, are derived from fossil shells of planktic foraminifera; minute, single-celled organisms which are ubiquitous in the World’s Oceans. A characteristic feature of foraminifera is their sand-grain-sized calcium carbonate (calcite) shell which is eventually deposited on the seafloor where it can be preserved for tens of millions of years. Fossil shells can be recovered from drill cores, and subsequent analyses of their chemical or isotopic composition allows for the reconstruction of the water temperature (and other parameters) that persisted while the shells were formed. This approach to reconstruct paleotemperatures works very well outside the polar regions. One of the main objectives of this study is to assess whether certain characteristics of the foraminifera species Neogloboquadrina pachyderma, which is the only abundant foraminifera in high latitude sediments, contribute to the challenges to deduce robust paleorecords from the Polar North Atlantic.
Pilot data acquired for this study indicate that the so-called inner ‘lamellar calcite’ of fossil N. pachyderma shells, which is the early generation of calcite that forms in the upper water column (typically warmer temperatures), is often largely dissolved, whereas the outer ‘crust calcite’, which forms in much deeper waters and thus typically reflects colder temperatures, is still well preserved. Interestingly, this loss of the lamellar calcite is not visible from the outside when the shells are inspected by microscope. Thus, it is likely that shells with dissolved lamellar calcite were selected for analysis in many prior studies, which may have biased the calculated temperatures towards the cooler, deeper waters where the remaining outer crust was formed. Fortunately, we have found that shells with well-preserved lamellar calcite are typically co-occurring with those featuring poor preservation.
The objective of this study is to develop new screening methods to carefully select N. pachyderma shells featuring well-preserved lamellar calcite. Subsequently, the lamellar calcite, which has formed near the sea surface, will be analyzed by laser-ablation for its Magnesium/Calcium (Mg/Ca) ratio, which can be converted to the water temperatures that persisted in the past when the foraminifera formed their shells. This is a significant deviation from conventional approaches that use the entire shell, whereby the information recorded in the lamellar calcite (near-surface signal) is mixed with the composition of the crust calcite (deep water signal). Thus, conventional analytical approaches may not capture the entire temperature variation at or near the sea surface. The analytical approach, which will be developed within the framework of this study, has the potential to significantly improve the veracity of paleorecords from the Polar North Atlantic, which is essential to gauge the response of this sensitive region to future greenhouse gas forcing. This new technique will further be tested on two sediment cores from the Polar North Atlantic, spanning the past ~26,000 years from the coldest temperatures of the last ice age to the modern, much warmer climate.
Testing geochemical proxy relationships under variable paleo-seawater chemical compositions