Description: Hydrothermal fluids expelled from the seafloor at high and low temperatures play pivotal roles in controlling seawater chemistry. However, the magnitude of the high temperature water flux of mid-ocean ridge axes remains widely disputed and the volume of low temperature vent fluids at ridge flanks is virtually unconstrained. Here, we determine both high and low temperature hydrothermal fluid fluxes using the chemical and isotopic mass balance of the element thallium (Tl) in the ocean crust. Thallium is a unique tracer of ocean floor hydrothermal exchange because of its contrasting behavior during seafloor alteration at low and high temperatures and the distinctive isotopic signatures of fresh and altered MORB and seawater. The calculated high temperature hydrothermal water flux is (0.17-2.93)*10**13 kg/yr with a best estimate of 0.72*10**13 kg/yr. This result suggests that only about 5 to 80% of the heat available at mid-ocean ridge axes from the crystallization and cooling of the freshly formed ocean crust, is released by high temperature black smoker fluids.The residual thermal energy ismost likely lost via conduction and/or through the circulation of intermediate temperature hydrothermal fluids that do not alter the chemical budgets of Tl in the ocean crust. The Tl-based calculations indicate that the low temperature hydrothermal water flux at ridge flanks is (0.2-5.4)*10**17 kg/yr. This implies that the fluids have an average temperature anomaly of only about 0.1 to 3.6 °C relative to ambient seawater. If these low temperatures are correct then both Sr and Mg are expected to be relatively unreactive in ridge-flank hydrothermal systems and this may explain why the extent of basalt alteration that is observed for altered ocean crust appears insufficient to balance the oceanic budgets of 87Sr/86Sr and Mg.

Description: Net ecosystem calcification rates (NEC) and net photosynthesis (NP) were determined from CO2 seawater parameters on the barrier coral reef of Kaneohe Bay, Oahu, Hawaii. Autosamplers were deployed to collect samples on the barrier reef every 2 hours for six 48-hour deployments, two each in June 2008, August 2009, and January/February 2010. NEC on the Kaneohe Bay barrier reef increased throughout the day and decreased at night. Net calcification continued at low rates at night except for six time periods when net dissolution was measured. The barrier reef was generally net photosynthetic (positive NP) during the day and net respiring (negative NP) at night. NP controlled the diel cycles of the partial pressure of CO2 (pCO2) and aragonite saturation state resulting in high daytime aragonite saturation state levels when calcification rates were at their peak. However, the NEC and NP diel cycles can become decoupled for short periods of time (several hours) without affecting calcification rates. On a net daily basis, net ecosystem production (NEP) of the barrier reef was found to be sometimes net photosynthetic and sometimes net respiring and ranged from -378 to 80 mmol m-2 d-1 when calculated using simple box models. Daily NEC of the barrier reef was positive (net calcification) for all deployments and ranged from 174 to 331 mmol CaCO3 m-2 d-1. Daily NEC was strongly negatively correlated with average daily pCO2 (R2 = 0.76) which ranged from 431 to 622 µatm. Daily NEC of the Kaneohe Bay barrier reef is similar to or higher than daily NEC measured on other coral reefs even though aragonite saturation state levels (mean aragonite saturation state = 2.85) are some of the lowest measured in coral reef ecosystems. It appears that while calcification rate and ?arag are correlated within a single coral reef ecosystem, this relationship does not necessarily hold between different coral reef systems. It can be expected that ocean acidification will not affect coral reefs uniformly and that some may be more sensitive to increasing pCO2 levels than others.