Welcome to the new version of CaltechAUTHORS. Login is currently restricted to library staff. If you notice any issues, please email coda@library.caltech.edu
Published April 15, 2012 | public
Journal Article

δ^(44/40)Ca variability in shallow water carbonates and the impact of submarine groundwater discharge on Ca-cycling in marine environments


Shallow water carbonates from Florida Bay, the Florida Reef Tract, and a Mexican Caribbean fringing reef at Punta Maroma were studied to determine the range of Ca-isotope variation among a cohort of modern carbonate producers and to look for local-scale Ca-cycling effects. The total range of Ca-isotope fractionation is 0.4‰ at Punta Maroma, yielding an allochem-weighted average δ^(44/40)Ca value of −1.12‰ consistent with bulk sediment from the lagoon with a value of −1.09‰. These values are virtually identical to bulk carbonate sediments from the Florida Reef Tract (−1.11‰) and from one location in Florida Bay (−1.09‰) near a tidal inlet in the Florida Keys. No evidence was found for the ∼0.6‰ fractionation between calcite and aragonite which has been observed in laboratory precipitation experiments. Combining these results with carbonate production modes and δ^(44/40)Ca values for pelagic carbonates taken from the literature, we calculate a weighted average value of −1.12 ± 0.11‰ (2σ) for the global-scale Ca-output flux into carbonate sediments. The δ^(44/40)Ca value of the input Ca-flux from rivers and hydrothermal fluids is –1.01 ± 0.04‰ (2σ_(mean)), calculated from literature data that have been corrected for inter-laboratory bias. Assuming that the ocean Ca cycle is in steady state, we calculate a δ^(44/40)Ca value of −1.23 ± 0.23‰ (2σ) for submarine groundwater discharge (SGD) on a global scale. The SGD Ca-flux rivals river flows and mid-ocean ridge hydrothermal vent inputs as a source of Ca to the oceans. It has the potential to differ significantly in its isotopic value from these traditional Ca-inputs in the geological past, and to cause small changes in the δ^(44/40)Ca value of oceans through time. In the innermost water circulation restricted region of northeastern Florida Bay, sediments and waters exhibit a 0.7‰ gradient in δ^(44/40)Ca values decreasing towards the Florida Everglades. This lowering of δ^(44/40)Ca is predominantly caused by local-scale Ca-inputs from SGD, which has a high Ca concentration (450 mg/L) and low δ^(44/40)Ca value (−0.96‰). Mixing calculations show that Ca inputs from SGD and surface water runoff from the Florida Everglades contribute between 8% and 60% of the dissolved Ca to the studied waters with salinities between 30 and 14, respectively. Similar degrees of circulation restriction between epeiric seas and oceans in the geological past may have also led to overprinting of sedimentary carbonate δ^(44/40)Ca values in nearshore regions of epeiric seas due to local-scale cycling of seawater through coastal carbonate aquifers. Local Ca-cycling effects may explain some of the scatter in δ^(44/40)Ca values present in the Ca-isotope evolution curve of Phanerozoic oceans.

Additional Information

© 2011 Elsevier Ltd. Received 1 July 2011; accepted in revised form 19 December 2011; available online 29 December 2011. Associate editor: Derek Vance. We thank Mosa Nasreen for assistance in the clean analytical laboratory, Dinka Besic for assistance in the mass spectrometry laboratory and Jim Rosen for electronics, mechanical and computer software support on the instruments used in this study. This work was made possible through financial support to C.H. from the Natural Science and Engineering Research Council of Canada (Discovery grant). Karla Panchuk and Andy Jacobson are thanked for comments and suggestions that helped to improve the presentation. Cathrin Hagey is thanked for editorial assistance, and Matt Fantle and three anonymous reviewers are thanked for reviewing the paper for the journal.

Additional details

August 19, 2023
October 17, 2023