How to be a Prehistoric Weatherman: Using n-alkanes as a Proxy for Holocene Climate and Hydrology, Southwest South Island, New Zealand

[摘要] The latitudinal position and strength of the Southern Hemisphere westerly winds(SHWW) plays a critical role in global CO2 air-sea flux and the distribution ofrainfall in the southern mid-latitudes. Strengthening and southward shiftingwesterlies are thought to be reducing the efficiency of the Southern Ocean carbon sink, which has direct implications for modern atmospheric CO2 concentrations. Southern New Zealand intersects the northern margin of the SHWW belt,where a direct relationship exists between annual precipitation and zonal windspeeds. Reconstructing past hydrological variations from the southwest of theSouth Island (SWSI) can provide a regional record of climatic response to changing SHWW. A 5.4 m sediment core spanning the last 10.8 ka was recovered fromSouth Mavora Lake, Southland. Magnetic susceptibility, bulk organic C and Nisotopes and concentrations, and lipid biomarker (C21-C33 n-alkanes) concentrations, distributions, and hydrogen isotope values (δD) serve as proxies for changein lacustrine productivity, relative proportions of terrestrial and aquatic input,and hydrology. Modern SWSI meteoric water isotope values (δ18O, δD) collectedover a 12-month period, and meteorlogical station data, show orographic rainoutand air temperature are the primary drivers of hydrological isotope compositionin SWSI. Downcore interpretation of data suggests a period of increased precipitation, rapid warming, and greater terrestrial input from 10.8-9.0 ka, likely corresponding to weaker westerly influence over SHWW. From 9.0-7.0 ka, decreasingδDn-alkanes shows gradual cooling, δ13C and ACL suggest increased aquatic productivity, and stratigraphy shows an increase in storm strength. From 7.0-5.1 kaδDn-alkanes and δ13C are characteristic of a relatively stable temperate climate,Paq and C/N ratios suggest a relatively humid environment, and stratigraphyshowed an increase of storm events. From 5.1-3.6 ka δDn-alkanes showed a largecold excursion followed by gradual warming, Paq and stratigraphy reflected a significant increase in storm event frequency and strength, and an increase in ACLreflected the expansion of cool-moist Nothofagus menziesii into the region. From3.6 ka to present δDn −alkanes showed a cooling trend to present day, likely relatedto strengthening of the SHWW, and low amplitude and frequency variation inPaq and decreased storm events signaled a gradual decrease in precipitation tomodern day conditions.

[发布机构] University of Otago

[效力级别] deuterium

[关键词] n-alkanes;deuterium;compound-specific;Geochemistry;New Zealand;biomarker;paleoclimate;southern hemisphere westerly winds;Mavora Lake;South Island