Holocene ice-ocean interactions in the Ross Sea and Adélie Land regions

Abstract

<p>Antarctica’s coastal margins are areas of complex oceanographic, atmospheric, and biological interactions. Characterising interactions and change in these regions is important for several reasons. First, variations in sea-ice extent, primary productivity, ocean circulation, and bottom water formation are important components of global heat distribution, the carbon cycle, and climate system. Second, multi-decadal to sub-decadal climate oscillations are inferred to affect sea ice and primary productivity in these areas, yet these influences are hard to quantify due to spatially and temporally limited data sets. Lastly, ice sheet margins resting on continental shelves with retrograded slopes, such as in the Ross Sea and along the Wilkes-Adélie Land Margin, are highly sensitive to climate and oceanographic fluctuations and future warming. Wind-driven changes in ocean currents that transfer heat on to Antarctica’s continental shelf, are inferred to be the primary driver of ice shelf thinning and grounding line retreat following the Last Glacial Maximum, and are also leading to freshening of surface waters along Antarctica’s continental margins. As the Ross Sea and Wilkes-Adélie Land Margin are thought to produce up to 25% of modern Antarctic Bottom Water and are major carbon sinks, developing high resolution paleoclimate records in these areas critically supports futureclimate projections.</p> <p>Two new high-resolution paleoclimate records allow for the reconstruction of ice-ocean-atmosphere interactions throughout the last ⇠11,400 years in the Ross Sea and Adélie Land regions. The RICE ice core was drilled at Roosevelt Island at the northeastern edge of the Ross Ice Shelf. The high accumulation rate (⇠20 cm water equivalent per year) at the site provides an annually resolved record of ice-ocean-atmosphere interactions in the Ross Sea. Downstream from, and thus influenced by the Ross Sea, lies the Adélie Basin where IODP sediment core U1357B was drilled. Sedimentation rates at this location are higher than any other published marine sediment record and average ⇠2 cm/year throughout the Holocene. This creates a unique opportunity to develop a near-annually resolved record which captures biologically influenced sedimentation from one of Antarctic’s largest coastal polynyas. The focus of this PhD is to first develop and interpret the IODP U1357B Holocene record. The second is to integrate this record with the RICE ice core in order to understand how each of these regions evolved during the Holocene, and to identify any interactions between these oceanographically linked sites. Using X-ray Computed Tomography, and supported by X-ray fluorescence data, lipid biomarkers, and other physical properties, a record of near annual biogenic bloom events is developed and linked to changing environmental conditions throughout the Holocene. Baseline shifts in laminae frequency correspond with changes in El Niño-Southern Oscillation (ENSO) frequency, as noted in other Holocene paleoclimate records. The precise assessment of this relationship is complicated by the fact that the Southern Annular Mode (SAM) and Indian Ocean Dipole (IOD), which moderate the impact of ENSO on Adélie Land, are not yet well defined in the paleoclimate record. In addition, the presence of coastal sea ice appears to play an important role in modulating the oceanographic and biological response to ENSO forcing. This suggests the sea ice state acts as a tipping point in the system to modulate low-to-high latitude teleconnections.</p> <p>New analysis on previously measured grain size distributions of light and dark laminae, in conjunction with examination of sediment advection rates, and laminae thickness data indicate that these distributions provide a relative estimate of current speed throughout the Holocene. These data indicate wind-driven currents are a primary control on laminae deposition and help drive bloom events through upwelling of nutrients. Measurement of major ions (Cl- , NO3- , SO42- , MSA- , Na+ , K+ , Mg2+, Ca2+ ) in the RICE ice core from the Early Holocene (⇠10.6 to 7.3 ka BP) are used to characterize aerosol delivery at the RICE site, and provide context for the interpretations of the continuous flow analysis Ca2+ record, which has been measured for the entire Holocene. The RICE continuous flow analysis Ca2+ and deuterium isotope record are then used in conjunction with data from the Taylor Dome ice core, Talos Dome ice core, and the Adélie Basin sediment core to track changes in atmospheric circulation and biological productivity as the Ross Ice Shelf retreated to its modern-day position. Two significant events are recorded in all records at ⇠8.9-8.5 ka BP and ⇠5-4.5 ka BP, and are linked to the reorganization of atmospheric transport pathways and the presence of cooler and fresher surface waters due to the formation of the Ross Ice Shelf cavity. Modification of atmospheric and oceanic systems as the Ross Ice Shelf grounding line retreated increased the sensitivity of Adélie Land to changes in the Ross Sea.</p> <p>Together, this thesis provides a new Holocene reconstruction of current speed and biological productivity in Adélie Land, East Antarctica. Comparison with the RICE ice core record and other Ross Sea records highlight the sensitivity of these regions to one another, and suggest future change in the Ross Sea will have large downstream impacts on primary productivity, Antarctic Bottom Water Formation, and the global carbon cycle.</p>