Controls on d234U in surface waters of the South Island of New Zealand

The development of new proxies with which to assess the interconnection of climate and continental weathering is important because of the role of silicate weathering in the long-term carbon cycle. A potential proxy of both modern and past weathering regimes is the U isotope ratio 234U/238U (expressed as d234U). River water is nearly always enriched in 234U, due to release of alpha-recoil mobilised 234U from continental rocks. High d234U values may be expected in areas where this recoil 234U can build up, or where fresh mineral surfaces are continually being exposed. The South Island of New Zealand is an ideal place to investigate the dominant controls on riverine d234U since it has a wide range of uplift rates and climate environments. We have analysed U concentration and d234U in more than 30 surface-water samples by MC-ICP-MS. These samples include rivers, lakes, groundwater and glacial melt. The concentration of U in both glacial ice and ground waters is low, so the dominant source of U in surface waters is from weathering. The d234U values are extremely variable (68 to > 3600) and display some of the highest values that have been observed. The bedrock chemistry is reasonably uniform so this range in d234U is not a function of changing lithology. The d234U values are, therefore, controlled only by the production rate of mobile 234U and its susceptibility to removal by river water. A clear positive relationship between river d234U and uplift rates is observed, suggesting that physical weathering is an important control on d234U. For instance, the samples with the highest d234U values (2800-3600) are from the zone of maximum uplift on the dry east coast, whereas low d234U values (~68-80) are observed further south where there is virtually no vertical uplift. Rainfall also plays a clear role as demonstrated by high rainfall on the west coast which causes lower riverine d234U when alpha-recoil mobilised 234U is quickly washed away. Both the U concentration and d234U are also altered by the lacustrine processes, particularly where glacial silt is present. Observed changes of d234U with time have been linked to past climate in a number of speleothem records. This study provides some support for the use of d234U as a proxy for rainfall, but also shows that d234U is extremely dependent on the rate of physical erosion.