Icebergs at the Agulhas Plateau through the Pleistocene: Accumulation, Provenance, and Interpretation of Ice-Rafted Debris

Jens.Gruetzner [ at ]


To understand how the relationship between ice sheets, oceans and climate will respond to continued anthropogenic warming, it is crucial to examine its evolution in the geological past. A key way that we can reconstruct glacio-marine processes is to study the deposition of sediment transported by free-floating ice to the open ocean. Here, we combine paleo-iceberg trajectory modelling with Pleistocene records of ice-rafted debris (IRD), lithogenic grain size distributions and clay mineralogy from the Indian-Atlantic Ocean Gateway, at the northern limit of the modern Sub-Antarctic Zone (SAZ), on the southern Agulhas Plateau (AP). The records we present are from a continuous splice of sediment core sites MD02-2588 and IODP Site U1475 41°25.61’S; 25°15.64’E, 2669 m water depth), spanning 0 – 1.65 Ma at an average of 1.5-kyr resolution. Given the distal location of the AP from the Antarctic continent, the sustained delivery of IRD is indicative of (likely massive) icebergs, traversing the Southern Ocean before depositing entrained sediment. Both our model analyses and IRD data show that SAZ iceberg rafting was generally higher during Pleistocene glacial periods, facilitated by increased transport and survivability. We characterise the signature of this IRD by mineralogical, geochemical, and grain size analysis. By determining the provenance of this material, it is possible to gain insight into the past export of icebergs from the Antarctic Ice Sheet, in particular identifying the response of marine-terminating glaciers to the range of climate conditions associated with Pleistocene glacial-interglacial cycles. SEM and EDS analysis of the IRD reveal mineralogies indicative of basement crystalline rock, and an absence of volcanic glass. This demonstrates an Antarctic origin for the sediment, as opposed to volcanic inputs from sub-Antarctic island arcs. Furthermore, the presence of garnet bearing an Almandine end-member signature indicates that the IRD deposited on the AP are of Weddell Sea origin. This is coherent with modelled iceberg trajectories showing a high export of icebergs from the Weddell Sea gyre into the Antarctic Circumpolar Current. An equatorward expansion of the SAZ likely increased the proximity of iceberg trajectories to the AP as well as improving the survivability of icebergs through surface cooling. We suggest that this process plays an important role in global climate by modulating the distribution of the SAZ freshwater budget and influencing the mode and intermediate water masses that connect the Southern Ocean to the upper limb of Atlantic Overturning Circulation.

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AGU Fall Meeting 2019, 09 Dec 2019 - 13 Dec 2019, San Francisco, U.S.A..
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Starr, A. , Hall, I. R. , Hemming, S. R. , Bigg, G. R. , Barker, S. , van der Lubbe, J. , Sierra, A. C. , Berke, M. A. , Gruetzner, J. , Jimenez, F. J. , Knorr, G. , LeVay, L. J. and Lathika, N. (2019): Icebergs at the Agulhas Plateau through the Pleistocene: Accumulation, Provenance, and Interpretation of Ice-Rafted Debris , AGU Fall Meeting 2019, San Francisco, U.S.A., 9 December 2019 - 13 December 2019 .

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