C. R. Riesselman

Sources and downstream variation of surface water chemistry in the dammed Waitaki catchment, South Island, New Zealand

ABSTRACT This article presents the results of a pilot study exploring downstream changes in water chemistry in the Waitaki catchment, which drains the eastern slopes of the Southern Alps. The headwaters of this catchment are glaciated, and both natural glacial lakes and hydroelectric reservoirs occur in the catchment. The dominant lithology is calcite-poor quartzo-feldspathic metasedimentary rock. We sampled surface waters from streams and the inflow and outflow of lakes in the catchment on four occasions over the course of 1year, at c. 3-monthly intervals between summer 2013 and spring 2014. We also sampled ice released from the terminus of the Tasman Glacier. Small but measurable variations were observed in the major element (Ca, Mg, K, Na, Si), trace element (Al, Fe, Rb, Sr) and Sr isotopic composition of the surface waters in the catchment. The composition of the waters is interpreted to reflect the change from calcite-dominated weathering near the top of the catchment toward more silicate weathering down the catchment that has been observed in other studies. The composition of samples collected from the uppermost part of the catchment displays more temporal variability than those samples collected from dammed lakes, suggesting that seasonality is dampened by anthropogenic modification of the catchment.

Ice loss from the East Antarctic Ice Sheet during late Pleistocene interglacials

Understanding ice sheet behaviour in the geological past is essential for evaluating the role of the cryosphere in the climate system and for projecting rates and magnitudes of sea level rise in future warming scenarios1–4. Although both geological data5–7 and ice sheet models3,8 indicate that marine-based sectors of the East Antarctic Ice Sheet were unstable during Pliocene warm intervals, the ice sheet dynamics during late Pleistocene interglacial intervals are highly uncertain3,9,10. Here we provide evidence from marine sedimentological and geochemical records for ice margin retreat or thinning in the vicinity of the Wilkes Subglacial Basin of East Antarctica during warm late Pleistocene interglacial intervals. The most extreme changes in sediment provenance, recording changes in the locus of glacial erosion, occurred during marine isotope stages 5, 9, and 11, when Antarctic air temperatures11 were at least two degrees Celsius warmer than pre-industrial temperatures for 2,500xa0years or more. Hence, our study indicates a close link between extended Antarctic warmth and ice loss from the Wilkes Subglacial Basin, providing ice-proximal data to support a contribution to sea level from a reduced East Antarctic Ice Sheet during warm interglacial intervals. While the behaviour of other regions of the East Antarctic Ice Sheet remains to be assessed, it appears that modest future warming may be sufficient to cause ice loss from the Wilkes Subglacial Basin.Studies of an Antarctic marine sediment core suggest that the East Antarctic Ice Sheet retreated in the vicinity of the Wilkes Subglacial Basin during extended warm periods of the late Pleistocene, when temperatures were similar to those predicted to occur within this century.