Benthic foraminiferal oxygen isotopes during the Last Glacial Maximum and last deglaciation : Paleoceanographic inferences from an isotope-enabled global ocean model

Abstract

Earth's climate system has been characterized by glacial-interglacial cycles over the last million years. Within the climate system, oceans play an important role, due to their large heat storage capacities and transport effects as well as being Eartha s largest carbon dioxide sink. The understanding and prediction of the past, present, and future climate change relies on reconstructions from climate proxies. A widely used paleoclimate proxy is the oxygen isotopic composition of foraminiferal calcite shells. It reflects both the temperature and the isotopic composition of seawater, due to the thermodynamic isotope fractionation that occurs during calcite precipitation. Besides being used to, e.g., illustrate changes in temperature and global ice volume as well as to identify past meltwater events or to reconstruct past seawater densities, it has been commonly used as a tracer to infer changes in Atlantic water masses associated with shifts in the ocean circulation. However, some uncertainties remain with this proxy, since its separation into its individual components regarding the temperature and isotopic composition of seawater is challenging. In this regard, an isotope-enabled ocean modeling approach can help to gain a better understanding of the oxygen isotopic signal recorded in foraminiferal shells and the causes of the observed variations under climate change. In this study, the three stable water isotopes H216O, H218O, and HDO were implemented into the Massachusetts Institute of Technology general circulation model, providing a tool, to directly compare to the wealth of the foraminiferal oxygen-isotope records and, thus, helping to infer water mass alterations due to changes in the thermohaline circulation under climate change scenarios. The stable water isotope scheme has extensively been evaluated against the modern isotopic composition of seawater on a global scale as well as against the oxygen isotope signal in foraminiferal calcite shells of plankton-tow data. The good agreement between the model and the observations implies that the here presented modeling approach can confidently be used for paleoceanographic and paleoclimatological related questions. Therefore, the ocean state during the Last Glacial Maximum was simulated to investigate the increase of the glacial-interglacial benthic oxygen isotope differences with water depth in the Atlantic Ocean. The smaller glacial-interglacial anomalies within the permanent thermocline are related to the glacial sea-level lowering of approximately 120 m, which brings the core locations closer to the surface. However, the enhanced glacial-interglacial anomalies in the deeper parts of the water column ( 2000 m) result from the expansion of the cold southern source water masses, implying that they are mainly temperature driven. Furthermore, this indicates that the enhanced vertical gradient at approximately 2000 m water depth corresponds to the glacial water-mass boundary in the Atlantic Ocean, coinciding with estimates as seen in benthic foraminiferal stable carbon isotope values. Under the influence of an additional freshwater flux into the North Atlantic Ocean an idealized Heinrich Event 1 model experiment was performed to investigate the causes for the decrease of the oxygen isotopic composition in benthic foraminifera in response to abrupt climate change and their resemblance with Antarctic ice-core records. Due to the freshwater perturbation the Atlantic Meridional Overturning circulation weakens, whereby a substantial warming within the intermediate water depths occurs. Thus, the oxygen isotopic composition corresponds almost entirely to an enhanced temperature signal, which is driven by an initial increase in horizontal heat transport followed by downward heat transport via vertical advection. Hence, the resemblance between the oxygen-isotope records in benthic foraminifera and Antarctic ice-core records is attributable to the modulation of Antarctic atmospheric temperatures and deep water temperatures by the strength of the Atlantic Meridional Overturning circulation. Quantifying the contribution of temperature and isotopic composition of seawater to the oxygen isotopic composition of foraminiferal calcite by using an isotope-enabled ocean model gives valuable findings and, thus, helps to advance the understanding and interpretation of this widely used proxy.