A Two-Year Automated Dripwater Chemistry Study in a Remote Cave in the Tropical South Pacific: Using [Cl-] as a Conservative Tracer for Seasalt Contribution of Major Cations

Wednesday, 17 December 2014
Darrel M Tremaine1, Daniel J Sinclair2, Heather M Stoll3, Carlos P Carvajal1, James R Sweeney1 and Robert M Sherrell1,4, (1)Rutgers Univ, Institute of Marine and Coastal Sciences, New Brunswick, NJ, United States, (2)Victoria University of Wellington, Wellington, New Zealand, (3)University of Oviedo, Oviedo, Spain, (4)Rutgers University, Department of Earth and Planetary Sciences, Piscataway, NJ, United States
Speleothem δ18O can be used as a terrestrial archive of climate variability, offering insight into rapid changes in atmospheric and oceanic circulation that impact terrestrial weather patterns [1]. However, separating δ18O ‘amount’, ‘source’, and ‘temperature’ effects is a challenge that requires the use of complementary rainfall proxies, such as trace element (X/Ca) ratios. Mg/Ca and Sr/Ca ratios are well understood in continental caves, providing information about wet-vs-dry hydrologic conditions within the epikarst [2], the strength and timing of cave ventilation [3], and evapotranspiration above the cave in arid climates [4]. However, there are few detailed investigations of maritime caves where dripwater X/Ca ratios are impacted by seaspray [5, 6].

Here we present results from a novel two-year automated dripwater monitoring program in a remote cave on the Island of Niue in the tropical south Pacific. High-resolution cave monitoring traditionally requires frequent site visits; monitoring this remote location required the development of new sampling strategies. The cave was instrumented for micrometeorology, dripwater δ18O and δD, and dripwater chemistry. Because of its maritime location, seaspray represents a significant contribution to karst waters. By using [Cl-] as a conservative tracer for seaspray input, we calculate that seasalt in the modern dripwater accounts for 20% of the Ca and Sr, and up to 85~95% of the Mg and Na, respectively.

Applying seasalt corrections, we find that increased Mg/Ca and Sr/Ca ratios correlate with enriched dripwater δ18O values, consistent with the amount effect and prior-calcite-precipitation (PCP). This short-term calibration suggests that when rainfall amount is driving speleothem chemistry in this cave, Mg/Ca and Sr/Ca ratios should strongly covary with δ18O, a result we will apply to interpretation of speleothem climate records being generated from Niue.

[1] Lachniet, (2009) QSR 28, 412-432; [2] Tremaine and Froelich (2013) GCA 121, 522-545; [3] Wong et al. (2011) GCA 75, 3514-3529; [4] Rutlidge et al. (2014) GCA 135, 217-230; [5] Partin et al. (2012) G3 13, Q03013; [6] Partin et al. (2013) G3 14, 3567-3585