Paleoenvironmental history of the Chilean Fjord region and the adjacent southeast Pacific over the last 60 kyr BP: A multiproxy analysis on high resolution sediment cores
The Chilean Fjord region offers a unique opportunity to study past ocenographic and environmental changes as it is the only land mass in the Southern Hemisphere intercepting the Southern Westerly Winds (SWW) which are of global importance. Three sediment cores from the Chilean Fjord Region and the adjacent southeast Pacific were used to study past sea surface temperature variations and their relationship to climate changes during most of the last glacial period and the Holocene. Core MD07-3128 was recovered at 53°S at the Pacific entrance of the Strait of Magellan. Carbon-14 ages and paleomagnetic chronology indicate that core MD07-3128 covers the last 60 kyr BP. Its high-sedimentation rates allow to depict millennial-scale sea surface temperature (SST) and paleoproductivity changes over the last 60 kyr BP. Cores MD07-3124 and JPC-42, on the other hand, were recovered at 51°S and 50°S, from Canal Concepción and Canal Wide, respectively. These cores were used to reconstruct the first continuos SST records over the entire Holocene. Alkenone-derived SST data from the southernmost record MD07-3128 (53°S) show that glacial temperatures were ca. 7°C colder than early Holocene SST values. Substantial millennial-scale variability of 2-3°C characterized the last glacial period, while a very strong warming of 7°C was recorded over the last deglaciation. A cooling trend culminating during the Last Glacial Maximum (LGM) has been recorded. This regional cooling has been related to the proximal location of the core site to the large Patagonian Ice Sheet (PIS) and related melt-water supply, at least during the LGM. An increase in the relatively abundances of C37:4 alkenones, a proxy for low salinity waters, support the idea of large meltwater input during this time interval. On millennial-time scales, changes in marine plankton productivity were also recorded. An increase in the siliceous plankton community over the calcareous characterized the colder intervals while during the warmer ones this pattern was reversed. These marine productivity changes can be associated with a northward shift of the Southern Ocean fronts and the opal belt during glacial cold periods and/or changes in the nutrient advection consistent with our lower planktonic foraminiferal d13C data. Interestingly, the siliceous community was comparatively lower around the LGM suggesting a reduced biological productivity probably due to enhanced meltwater input from the large PIS resulting in high surface water stratification. During the Holocene, cores JPC-42 and MD07-3124 show no evidence for an early Holocene climate optimum as previously described for the Chilean continental margin furthern north. In constrast, SST were warmer than present during most of the Holocene except for the prominent cooling centered at 5 kyr BP and the cooling trend recorded in the latest Holocene. the absence of an early Holocene warm phase in the inner fjord has been associated with a combination of factors including inflow of open marine waters due to lower sea-level, enhanced advection of colder and fresher inner fjord water, stronger westerly winds, and reduced local summer insolation. The fact that alkenones in the inner fjord core JPC-42 (located near to the southern Patagonian icefield) were only present after 8 kyr BP point to susbtantially reduced surface water salinities during the early Holocene, strongly consistent with increasinng precipitation and runoff in the core of the SWW
