APPLICATION OF STABLE CALCIUM ISOTOPES TO CA CYCLING AT THE LUQUILLO CRITICAL ZONE OBSERVATORY

Stable Ca isotopes are an emerging tracer of Ca cycling in terrestrial ecosystems. The δ44Ca of streamwater should reflect internal sources of Ca while the δ44Ca of the soil exchange pool should reflect the relative magnitude of external Ca inputs, Ca uptake by plants and Ca return to the soil pool. We are measuring stable Ca isotope ratios in streamwater and soil cation exchange pools at two watersheds (Bisley I and Rio Icacos) underlain by contrasting lithologies, which comprise the LCZO. Our preliminary data indicate a first order contrast in δ44Ca patterns between these tropical sites and temperate sites that have been studied previously. Most rocks (the ultimate source of Ca to ecosystems) have δ44Ca ≈ -1‰ relative to the seawater standard. In northeast US forests (New York, Massachusetts, New Hampshire), both soils and streamwater tend to be similar to or isotopically lighter than rock and atmospheric sources of Ca. In contrast, our data from the LCZO sites indicates soil pools and streamwaters are isotopically similar to or heavier than rocks. Sea salt aerosol is a external source of heavy Ca in some systems, but previous work based on ⁸⁷Sr/⁸⁶Sr and Sr/Ca seems to discount this as a significant Ca source at the LCZO. Instead this discrepancy may be related to fundamental differences in plant-soil Ca cycling. Furthermore, our preliminary data suggest differences between the litholigcally contrasting LCZO sites. At Bisley I (volcaniclastic), δ44Ca in the soil exchange pool is heaviest (-0.02‰) at 0-5cm and lighter (-0.71‰) at 5-10cm depth. Baseflow δ44Ca (-1.00‰) likely reflects Ca from deep weathering of primary minerals, while heavier δ44Ca (-0.69‰) at storm flow suggests significant export of soil Ca. At Icacos (granodiorite), shallow soil δ44Ca (-0.71‰ at 0-8cm) is not as light as Bisley. Baseflow δ44Ca at Icacos is similar to Bisley (-0.89‰), though does not change significantly during stormflow (-0.92‰). Isotopically heavy Ca (relative to inputs) in the near-surface exchange pool and in streamwater at Bisley may reflect a non-steady state Ca cycle where uptake of light Ca by plants is greater than Ca return to soil, leaving the soil Ca pool isotopically heavy. That Icacos δ44Ca values are similar to rock may reflect high Ca input fluxes relative to plant uptake, suppressing the impact of vegetation fractionation.