Abiotic and biotic carbonate structures in karst formations from the Yucatan peninsula; their records of past and present hydrochemistry

Abstract

Microbialites are sedimentary structures built by microorganisms through trapping and binding sediment and/or biological-mediated carbonate precipitation. Carbonate structures from four different locations in the Yucatan Peninsula have been documented and analyzed to determine origins and relationships with current and past hydrochemistry. Cenote Fatima had three carbonate structures: two at -13 m depth (CF1A, CF2A) and one at -27 m depth (CF3A) and they were in sheltered light conditions. Two samples were from La Concha (LC1A, LC2A) with a similar depth at -12 m and two more samples were at Hoyo Negro at -18 m (HN1A), and at -45 m (HN2A) which were in dark conditions. Samples from Xul Ha (XH1A and XH2A) next to Laguna Bacalar which has some of the largest freshwater microbialite formations in the world and both these locations were in full light conditions. S/Ca ratios in Xul Ha samples show a hydrogeochemical origin linked to the dissolution of gypsum from underground karst. Xul Ha had the highest recorded S and Sr levels and our results match previous geochemical research on stromatolites and thrombolites. Samples from Cenote Fatima had radiocarbon ages that matched past water levels that have been recorded in Hoyo Negro and Hells Bells showing a drop in δ 13C of ≈3.5‰ between ≈7600 – ≈9000 cal. yr. BP. Cenote Fatima also shows a narrow growth window in the sunlit upper water column (3-4m) indicating a biogenically influenced origin. Hoyo Negro samples were shown to match the geochemistry of Holocene aquifer based on calcite raft core records and were deemed to be more abiotic based on prior research on Hells Bells in Cenote Zapote. Based on radiocarbon ages they thought to have formed during Stage 5e (125,000 yr. BP).