Abstract
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 of
rainfall in the southern mid-latitudes. Strengthening and southward shifting
westerlies 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 wind
speeds. Reconstructing past hydrological variations from the southwest of the
South 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 from
South Mavora Lake, Southland. Magnetic susceptibility, bulk organic C and N
isotopes and concentrations, and lipid biomarker (C21-C33 n-alkanes) concentrations, distributions, and hydrogen isotope values (δD) serve as proxies for change
in lacustrine productivity, relative proportions of terrestrial and aquatic input,
and hydrology. Modern SWSI meteoric water isotope values (δ18O, δD) collected
over a 12-month period, and meteorlogical station data, show orographic rainout
and air temperature are the primary drivers of hydrological isotope composition
in 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, decreasing
δDn-alkanes shows gradual cooling, δ13C and ACL suggest increased aquatic productivity, and stratigraphy shows an increase in storm strength. From 7.0-5.1 ka
δDn-alkanes and δ13C are characteristic of a relatively stable temperate climate,
Paq and C/N ratios suggest a relatively humid environment, and stratigraphy
showed an increase of storm events. From 5.1-3.6 ka δDn-alkanes showed a large
cold excursion followed by gradual warming, Paq and stratigraphy reflected a significant increase in storm event frequency and strength, and an increase in ACL
reflected the expansion of cool-moist Nothofagus menziesii into the region. From
3.6 ka to present δDn −alkanes showed a cooling trend to present day, likely related
to strengthening of the SHWW, and low amplitude and frequency variation in
Paq and decreased storm events signaled a gradual decrease in precipitation to
modern day conditions.