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

Abstract:

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