Timothy C.W. Ku

The carbonate system geochemistry of shallow groundwater–surface water systems in temperate glaciated watersheds (Michigan, USA): Significance of open-system dolomite weathering

We present here a field geochemical study of controls on carbonate weathering within rapidly circulating, shallow groundwater–surface water systems in the glaciated mid-continent region. Groundwaters and surface waters in three watersheds spanning the Upper to Lower Peninsulas of Michigan consist of Ca 2+ -Mg 2+ -HCO 3 − solutions derived from the open-system dissolution of calcite and dolomite in soils developed on mixed mineralogy glacial drift. The thermodynamic stabilities of calcite and dolomite both decrease with decreasing temperature, with dolomite more strongly affected. Thus, the low mean annual temperature of these temperate weathering environments maximizes the absolute solubility of dolomite as well as its solubility relative to calcite. Many groundwaters in the study area approach equilibrium with respect to the more soluble dolomite and are moderately supersaturated with respect to calcite. Groundwaters in each watershed have distinct and relatively narrow ranges of carbon dioxide partial pressure ( P CO 2 ) values, which increase significantly from north to south (log P CO 2 of −3.0 to −2.2 atm), suggesting that there are landscape-level differences in carbon transformation rates in soil weathering zones. Increases in weathering-zone P CO 2 values produce HCO 3 − concentrations that vary by a factor of five, but the Mg 2+ /Ca 2+ and Mg 2+ /HCO 3 − ratios of all groundwaters are similar, suggesting relatively constant weathering input ratios of calcite and dolomite. Although surface waters commonly are between 2 and 10 times supersaturated with respect to calcite, the Mg 2+ /HCO 3 − ratios of surface waters are very close to initial groundwater values, suggesting that back precipitation of calcite is not a significant process in these systems. The enhanced solubility of dolomite at low temperatures coupled with the landscape-level differences in carbon cycling suggest that temperate-zone weathering reactions in glaciated terrains are significant contributors to continent-scale fluxes of both Mg 2+ and HCO 3 − .

Carbonate Preservation in Shallow Marine Environments: Unexpected Role of Tropical Siliciclastics

Coordinated taphonomic, geochronologic, and geochemical studies of bivalve death assemblages and their sedimentary environments of San Blas, Caribbean Panama, permit us to identify the major factors controlling skeletal degradation in mixed carbonate‐siliciclastic tropical shelf sediments. Ten sites were studied along environmental gradients including water nutrients, grain size, and sediment chemistry (carbonate, organic carbon, and reactive iron contents). Taphonomic data were derived from naturally occurring bivalve death assemblages and experimentally deployed specimens of Mytilus edulis and Mercenaria mercenaria to determine environmental controls on types and intensities of postmortem damage to skeletal hardparts and to quantify short‐term rates of damage accrual. Death assemblage shells were dated using 14C and amino acid racemization techniques to examine shell persistence, scales of time averaging, and long‐term rates of damage accrual, including correlations between shell damage and shell age. Pore water and sediment geochemical analyses were used to determine the pathways and extent of early diagenetic change in the different sediment–pore water environments. We found that carbonate shell preservation is enhanced in dominantly siliciclastic sediments compared to dominantly carbonate sediments. The most important factors limiting the postmortem persistence of shell material are (1) exposure above the sediment‐water interface, which is enhanced in coarser‐grained carbonate sediments and permits attack by bioeroders and encrusters; (2) the availability of abundant reactive iron mineral phases in the sediments, which promotes supersaturated pore waters and limits acid production; and (3) shell microstructure (rather than mineralogy), particularly organic content that is the focus of intense microbial attack. Thus, there is significant potential for enhanced carbonate shell preservation in areas receiving ferric‐rich tropical weathering products, which are common in much of the tropics today and are associated with subduction systems in the geologic past. This suggests that paleodiversity estimates from carbonate tropical settings are minima and that siliciclastic settings are probably underestimated regions for carbon burial, given the large proportion of tropical shelf area characterized by such conditions and the relatively high proportional capture there of local carbonate production.

Seagrass blue carbon dynamics in the Gulf of Mexico: Stocks, losses from anthropogenic disturbance, and gains through seagrass restoration

Seagrasses comprise a substantive North American and Caribbean Sea blue carbon sink. Yet fine-scale estimates of seagrass carbon stocks, fluxes from anthropogenic disturbances, and potential gains in sedimentary carbon from seagrass restoration are lacking for most of the Western Hemisphere. To begin to fill this knowledge gap in the subtropics and tropics, we quantified organic carbon (Corg) stocks, losses, and gains from restorations at 8 previously-disturbed seagrass sites around the Gulf of Mexico (GoM) (n=128 cores). Mean natural seagrass Corg stocks were 25.7±6.7MgCorgha-1 around the GoM, while mean Corg stocks at adjacent barren sites that had previously hosted seagrass were 17.8MgCorgha-1. Restored seagrass beds contained a mean of 38.7±13.1MgCorgha-1. Mean Corg losses differed by anthropogenic impact type, but averaged 20.98±7.14MgCorgha-1. Corg gains from seagrass restoration averaged 20.96±8.59Mgha-1. These results, when combined with the similarity between natural and restored Corg content, highlight the potential of seagrass restoration for mitigating seagrass Corg losses from prior impact events. Our GoM basin-wide estimates of natural Corg totaled ~36.4Tg for the 947,327ha for the USA-GoM. Including Mexico, the total basin contained an estimated 37.2-37.5Tg Corg. Regional US-GoM losses totaled 21.69Tg Corg. Corg losses differed significantly among anthropogenic impacts. Yet, seagrass restoration appears to be an important climate change mitigation strategy that could be implemented elsewhere throughout the tropics and subtropics.