A multi-proxy lacustrine record of Holocene climate change in SW Patagonia, Chile

Abstract:

One of the largest uncertainties in the Holocene ocean-atmosphere carbon cycle is the role of the Southern Ocean (SO) and subpolar seas in controlling atmospheric CO₂ levels. The strength and position of the westerly winds is thought to control rates of CO₂ exchange as well as depth of ventilation. Secondarily, through their control on the large scale geostrophic circulation, the westerlies influence the position of major ocean frontal boundaries as well as stratification in the Southern Ocean – additional controls on SO carbon uptake. However, little is known about westerly wind variability over the Holocene. Southern Patagonia is an ideal locality for addressing this uncertainty, as it is the only major landmass that extends into the southern westerly wind field. In particular, lake and fjord environments hold potential for reconstructing precipitation, which is closely correlated with westerly wind strength in this region. Here we present a multi-proxy lacustrine sediment core record from Lago Sarmiento (51.06˚S, 72.91˚W), a large, closed-basin lake in southwest Chilean Patagonia. We observe highly variable C:N ratios between ~12,500 and ~6,000 ¹⁴C yr BP, indicating recurrent transitions from grassland to forest. We measured a steady increase in the C:N ratio from ~6,000 ¹⁴C yr BP to present, indicating a sustained shift from grassland to the modern Nothofagus forest and more humid conditions. We observe a general decreasing trend in bulk carbonate δ¹⁸O since ~4,000 ¹⁴C yr BP, which we interpret as a progressive increase in westerly wind intensity and associated tendency towards positive water balance. Weight percent bulk carbonate varies greatly, with a strong decreasing trend in the early Holocene, followed by an increasing trend during the mid-Holocene, and a dip followed by a short rise in the late Holocene. Comparison with other records from southern Patagonia and Antarctica is improving our understanding of the forcing mechanisms driving changes in the southern hemisphere westerlies.