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

Stalagmite Mg/Ca and Sr/Ca ratios are commonly interpreted as proxies for past hydrologic conditions and are often used to supplement carbon and oxygen stable isotope records. While the processes that control these element ratios, including water-rock interaction, dripwater residence time, and upstream precipitation of calcite, are well understood in continental caves, there have been few investigations of dripwater Element/Ca (X/Ca) evolution in coastal marine caves where seasalt can have a strong influence on the incoming Mg/Ca ratio.We instrumented a marine cave on the remote South Pacific island of Niue to record daily cave microclimate, as well as weekly-integrated drip rates, dripwater oxygen and hydrogen isotopes, and dripwater chemistry over a period of twenty-two months. Using chloride as a conservative tracer for sea-spray, we calculate that seasalt input accounts for a large portion of dripwater Na, SO 4 , and Mg (89%, 93%, and 85% respectively) and a smaller portion of the Ca and Sr (19% and 17%). During the second year of this study a gradual decrease (by $18%) in dripwater chlorinity was observed, suggesting that an epikarsthosted seasalt aerosol inventory was being diluted over time. Minor element to calcium ratios for B, K, Cl, SO 4 , Mg, Na, Sr, and Fe all strongly covary over the observation period, suggesting that although sea-spray plays a significant role in modulating incoming drip chemistry, prior calcite precipitation (PCP) dominates chemical evolution within the epikarst. During a prolonged drought episode, evaporative enrichments in dripwater dD and d 18 O (+4‰ and 0.5‰, respectively) were observed to coincide with increased cation and anion concentrations, strong Ca removal via PCP, and increases in Sr/Ca and Mg/Ca ratios (28% and 34%, respectively), suggesting that concomitant enrichment in speleothem d 18 O and X/Ca ratios may be interpreted as multi-proxy evidence for dry climate conditions. We use modern dripwater chemistry and empirical water-calcite distribution coefficients to predict a range of stalagmite X/Ca ratios. We then forward model a number of scenarios that could modulate stalagmite chemistry, including increased/decreased seasalt input and changing dripwater flow path through calcite, dolomite, and aragonite bedrock. One major implication from this study is that even if PCP and flow path lithology remain constant over time, changing seasalt input can drive stalagmite Mg/Ca and Sr/Ca ratios away from PCP-controlled covariation, and lead to strongly varying http://dx. ScienceDirect Geochimica et Cosmochimica Acta 184 (2016) 289-310Sr/Mg ratios. Thus in order to interpret coastal cave stalagmite X/Ca records accurately, it is necessary to estimate seasalt input and analyze parent drip and bedrock chemistry to quantify the influence of each contributing process.

Pacific climate reflected in Waipuna Cave dripwater hydrochemistry

“…As highlighted in Figure 7,  18 O values vary minimally around a mean dripwater  18 O value of -5.6 ‰, which is only slightly lower than the average rainwater value (-5.15 ‰, Keller 2014). This similarity indicates rapid mixing of freshly infiltrating water with older water in the epikarst, as also found in earlier studies from the Waitomo district (Williams 335 and Fowler, 2002) and elsewhere (Mattey et al, 2008;Tremaine et al, 2016;Breitenbach et al, 2019). The observed buffering of dripwater towards the mean rainwater  18 O value suggests that speleothem  18 O ratios can be expected to reflect (multiannual to multi-decadal changes in rainfall isotope geochemical patterns.…”

“…Here we report elemental composition data for those components typically influenced by prior calcite precipitation (PCP), i.e. Ca concentration (Fairchild et al, 2000) and Mg/Ca and Sr/Ca ratios (Tremaine et al, 2016). Dripwater Ca concentrations range from 0.72 to 2.27 molL -1 , Mg/Ca varied from 17.77 to 66.55 and Sr/Ca varied from 0.24 to 1.116 (mmol mol -1 ).…”

“…Cave monitoring studies in the south-western Pacific regions have documented strong ENSO signals in Australia (Tadros et al, 2016), Borneo (Moerman et al, 2014) and on Niue Island in the central Pacific (Tremaine et al, 2016). In New Zealand, 105 however, the number of comparable monitoring studies is still limited.…”

Trade winds drive pronounced seasonality in carbonate chemistry in a tropical Western Pacific island cave—Implications for speleothem paleoclimatology

“…To our knowledge, tropical cave p CO 2 has only been reported previously four times in the literature. The two published time series of cave p CO 2 at tropical cave sites—from Yok Balum Cave, Belize (16.2°N, 89.1°W) [ Ridley et al ., ] and Niue Cave, Niue Island (19.1°S, 169.9°W) [ Tremaine et al ., ]—show p CO 2 varying seasonally between near ambient p CO 2 and ∼800 ppm. Spot measurements made in several caves across Boreno in June 2006 never exceeded 800 ppm [ Partin et al ., ], and a range of 1180–2720 ppm measured during five field trips at unspecified times during a 15 month period was reported at São Bernardo Cave, Brazil (13.8°S, 46.4°W) [ Moquet et al ., ].…”

“…If speleothem calcite is not deposited during the wet season, the ability to distinguish changes in moisture conditions through speleothem calcite trace elements is limited. Instead, speleothem trace elements records at this coastal cave site may be primarily controlled by other processes, including sea spray [ Tremaine et al ., ].…”