Warming drives dissolved organic carbon export from pristine alpine soils.

Despite decades of research, the influence of climate on the export of dissolved organic carbon (DOC) from soil remains poorly constrained, adding uncertainty to global carbon models. The limited temporal range of contemporary monitoring data, ongoing climate reorganisation and confounding anthropogenic activities muddy the waters further. Here, we reconstruct DOC leaching over the last ~14,000 years using alpine environmental archives (two speleothems and one lake sediment core) across 4° of latitude from Te Waipounamu/South Island of Aotearoa New Zealand. We selected broadly comparable palaeoenvironmental archives in mountainous catchments, free of anthropogenically-induced landscape changes prior to ~1200 C.E. We show that warmer temperatures resulted in increased allochthonous DOC export through the Holocene, most notably during the Holocene Climatic Optimum (HCO), which was some 1.5-2.5 °C warmer than the late pre-industrial period-then decreased during the cooler mid-Holocene. We propose that temperature exerted the key control on the observed doubling to tripling of soil DOC export during the HCO, presumably via temperature-mediated changes in vegetative soil C inputs and microbial degradation rates. Future warming may accelerate DOC export from mountainous catchments, with implications for the global carbon cycle and water quality.

The impact of seasonal and event-based infiltration on transition metals (Cu, Ni, Co) in tropical cave drip water.

RATIONALE: The first-row transition metals Cu, Ni, and Co show a strong binding affinity to natural organic matter. Compared to dissolved elements and stable water isotopes, they may be transported rapidly through the soil and host rock into caves in response to infiltration events. This study aims to assess the potential of transition metal ratios as indicators for infiltration changes in response to the seasonal and/or event-based rainfall variation. METHODS: We developed a protocol to analyze Cu, Ni, and Co in the cave drip water using collision cell ICP-QMS without extensive sample pretreatment. The high Ca matrix leads to significant isobaric interferences on all isotope masses. Our method includes a correction of these matrix effects and yields results with comparable accuracy and reproducibility to other published methods. We applied this protocol to drip water samples from Larga Cave (Puerto Rico) covering at least two full annual cycles between 2014 and 2019 on a bimonthly scale. RESULTS: The analysis of external reference materials yielded a reproducibility between 4.7% and 9.2% (relative standard deviation), validating the accuracy of the matrix correction method. The limit of detection is <0.04 ppb for Cu, <0.02 ppb for Ni, and <0.008 ppb for Co. The analysis of drip water samples from Larga Cave reveals pronounced changes of several orders of magnitude in all Element (El) to Ca, Cu/Ni, and Cu/Co ratios in response to seasonal infiltration changes. In addition, we observe a partly even stronger response after major tropical storms and heavy precipitation events of the period of record, for example, tropical storm "Bertha" (2014) and the category 5 hurricanes "Irma" and "Maria" (both 2017). CONCLUSIONS: Transition metal ratios can be accurately measured in cave drip waters with high Ca matrix. At our tropical site, these are promising tracers of infiltration changes in response to changes in the amount of rainfall, providing the first step toward tropical cyclone reconstruction using trace elements in speleothems.