Early Holocene Antarctic climate variability - Drivers and consequences as captured by major ions in the Roosevelt Island Climate Evolution (RICE) ice core

In a warming world, rapid disintegration of the West Antarctic Ice Sheet (WAIS) remains a primary uncertainty in the Intergovernmental Panel on Climate Change (IPCC) sea level rise projections. A deep ice core was drilled at the northeastern edge of the Ross Ice Shelf as part of the Roosevelt Island Climate Evolution (RICE) project. The site is located in a major drainage pathway of the marine based WAIS and the primary focus of the RICE project is to provide new insights into our understanding of the stability of the Ross Ice Shelf (RIS) in a warming climate and associated sea-level rise contributions of the WAIS.

Due to its coastal location Roosevelt Island is sensitive to ice-ocean-atmosphere interactions and is characterised by high snow accumulation of 20 cm per year affording a high resolution ice core record. This PhD project focuses on the Early Holocene major ion record, comprising measurements of the anions Cl– , NO – 3 , SO 2 – 4 and MSA– and the cations Na+, K+, Mg2+ and Ca2+ as they can provide insight into changing atmospheric circulation patterns, sea ice conditions and primary productivity. This time period is particularly interesting as recent results from marine studies, integrated with the latest generation of ice sheet models, suggest that the majority of deglacial retreat of Ross Sea grounded ice occurred during the Early Holocene.

Using chemical signatures in a seasonally resolved section of the core the RICE sea salt (ss) aerosol (e.g. ssNa+) is shown to be sensitive to sea ice extent in the Ross Sea. The biogenic sulphur species methanesulphonic acid (MSA– ) is used to investigate sea ice extent and primary productivity in the Ross Sea region. Primary productivity responds to availability of light and nutrients and this makes MSA– a sensitive recorder of sea ice conditions and perhaps upwelling of nutrient rich deep waters. The common centennial scale variability in the RICE MSA– and sea salt proxy during the early Holocene optimum is interpreted as a reflection of the thermohaline circulation’s internal variability driving seasonal sea ice and open water conditions as well as primary productivity in the Ross Sea.

The comparison of the Mid-Early Holocene (12-6 thousand years before present, ka BP) RICE record to available regional ice core records highlights the sensitivity of RICE to changing oceanic conditions in the proximity to Roosevelt Island and provides new insights into our understanding of Holocene RIS grounding line retreat: The most significant increase in both sea salt aerosol (ssNa+) and primary productivity (MSA– ) occurs from 11.3 to 9 ka BP. During this time regional non-sea salt Ca2+ records from existing ice cores in the Ross Sea region (WAIS Divide, Siple Dome and TALDICE) suggest a period of reduced westerly wind intensity. Supported by evidence from marine records and modelling studies this Early Holocene baseline shift is linked to the retreat of grounded ice in the Ross Sea embayment. As the grounding line retreats, new space is created for the establishment of a seasonal sea ice cycle with seasonally open water conditions in the central/eastern Ross Sea facilitating enhanced sea salt aerosol formation (ssNa+), from both sea ice surfaces and the open ocean and increased phytoplankton activity in summer.