Do drifts deposited adjacent to carbonate platforms record the signal of global carbon isotopic values?

This study investigates the δ¹³C values of Middle Miocene–Modern drift deposits and periplatform sediments in the Maldives and compares these data with the global δ¹³C values derived from bulk oceanic sediments and foraminifera. This comparison reveals that while the δ¹³C values of the early Miocene periplatform sediments in the Maldives appear to track the global record of δ¹³C values, including increases associated with the Oligocene–Miocene boundary as well as the variations within the Monterey Event, the correlation with the Monterey Event may be coincidental. It is suggested that variations in δ¹³C values do not reflect changes in oceanic dissolved inorganic carbon, but instead pulses of sediment arising from platform progradation that contribute carbonates with elevated δ¹³C values derived from the adjacent shallow‐water atolls. This conclusion is supported both by correlations between the seismic sequence architecture and the δ¹³C values which document progradation of ¹³C‐rich platform sediments, and also by the continuation of the interval of ¹³C‐rich sediments past the end of the Monterey Event at 13 Ma within the drift.     

Controls on nutrient and phytoplankton dynamics during normal flow and storm runoff conditions, southern Kaneohe Bay, Hawaii

Fluvial effects on nutrient and phytoplankton dynamics were evaluated in southern Kaneohe Bay, Oahu, Hawaii. Fluvial inputs occurred as small, steady baseflows interrupted by intense pulses of storm runoff. Baseflow river inputs only affected restricted areas around stream mouths, but the five storm events sampled during this study produced transient runoff plumes of much greater spatial extent. Nutrient loading via runoff generally led to an increase of the phytoplankton biomass and gross primary productivity in southern Kaneohe Bay, but the rapid depletion of nutrients resulted in a decline of the algal populations in the relatively short time of days. Under baseline conditions, water column primary productivity in southern Kaneohe Bay is normally nitrogen limited. Following storm events, the high ratio of dissolved inorganic nitrogen to dissolved inorganic phosphorus (DIN:DIP, 25–29) fluxes of runoff nutrients drove bay waters towards phosphorus limitation. A depletion of phosphate relative to DIN in surface waters was observed following all storm events. Due to high flushing rates, recovery times of bay waters from storm perturbations ranged from 3 to 8 d and appeared to be correlated with tidal range. Storm inputs have a significant effect on the water column ecosystem and biogeochemistry in southern Kaneohe Bay. The perturbations were only transient events and the system rapidly recovered to prestorm conditions.     

Tentative kinetic model for dolomite precipitation rate and its application to dolomite distribution

The ‘dolomite problem’ has a long history and remains one of the most intensely studied and debated topics in geology. Major amounts of dolomite are not directly forming today from seawater. This observation has led many investigators to develop geochemical/hydrologic models for dolomite formation in diagenetic environments. A fundamental limitation of the current models for the growth of sedimentary dolomite is the dearth of kinetic information for this phase, in contrast to that available for calcite and aragonite. We present a simple kinetic model describing dolomite growth as a function of supersaturation using data from published high temperature synthesis experiments and our own experimental results. This model is similar in form to empirical models used to describe precipitation and dissolution rates of other carbonate minerals. Despite the considerable uncertainties and assumptions implicit in this approach, the model satisfies a basic expectation of classical precipitation theory, i.e., that the distance from equilibrium is a basic driving force for reaction rate. The calculated reaction order is high (~ 3), and the combined effect of high order and large activation energy produces a very strong dependence of the rate on temperature and the degree of supersaturation of aqueous solutions with respect to this phase. Using the calculated parameters, we applied the model to well-documented case studies of sabkha dolomite at Abu Dhabi (Persian Gulf), and organogenic dolomite from the Gulf of California. Growth rates calculated from the model agree with independent estimates of the age of these dolomites to well within an order of magnitude. A comparison of precipitation rates in seawater also shows the rate of dolomite precipitation to converge strongly with that of calcite with increasing temperature. If correct, this result implies that dolomite may respond to relatively modest warming of surface environments by substantial increases in accumulation rate, and suggests that the distribution of sedimentary dolomite in the rock record may be to some extent a temperature signal.     

Dolomite Controls on Phanerozoic Seawater Chemistry

We investigate the potential role of dolomite as a long-term buffer on Phanerozoic seawater composition. Using a comprehensive model of Phanerozoic geochemical cycling, we show how variations in the formation rate of sedimentary marine dolomite have buffered seawater saturation state. The total inventory of inorganic carbon reflects the sum of fluxes derived from continental weathering, basalt-seawater exchange, alumino-silicate diagenesis (reverse weathering), and global deposition of calcium carbonate. Although these fluxes are approximately balanced, model results indicate that seawater saturation state is sensitive to the marine dolomite depositional flux. This conclusion is consistent with and constrained by independent proxy data for seawater ion ratios, paleo-atmospheric CO₂ concentrations, and paleo-pH data, and dolomite mass-age distribution through Phanerozoic time. Abundant research indicates that dolomite’s occurrence in marine sediments is sensitive to many factors: temperature, seawater composition, paleogeographic setting, continental organization, etc. Although the complexity of the process of dolomite formation prevents a complete understanding of the relative role of these factors, our model results clearly underscore the importance of this mineral in the chemical history of Phanerozoic seawater.     

Coastal Ocean Last Glacial Maximum to 2100 CO2-Carbonic Acid-Carbonate System: A Modeling Approach

Using coupled terrestrial and coastal zone models, we investigated the impacts of deglaciation and anthropogenic inputs on the CO₂–H₂O–CaCO₃ system in global coastal ocean waters from the Last Glacial Maximum (LGM: 18,000 year BP) to the year 2100. With rising sea level and atmospheric CO₂, the carbonate system of coastal ocean water changed significantly. We find that 6 × 10¹² metric tons of carbon were emitted from the coastal ocean, growing due to the sea level rise, from the LGM to late preindustrial time (1700 AD) because of net heterotrophy and calcification processes. This carbon came to reside in the atmosphere and in the growing vegetation on land and in uptake of atmospheric CO₂ through the weathering of rocks on land. It appears that carbonate accumulation, mainly, but not exclusively, in coral reefs from the LGM to late preindustrial time could account for about 24 ppmv of the 100 ppmv rise in atmospheric CO₂, lending some support to the “coral reef hypothesis”. In addition, the global coastal ocean is now, or soon will be, a sink of atmospheric CO₂. The temperature rise of 4–5°C since the LGM led to increased weathering rates of inorganic and organic materials on land and enhanced riverine fluxes of total C, N, and P to the coastal ocean of 68%, 108%, and 97%, respectively, from the LGM to late preindustrial time. During the Anthropocene, these trends have been exacerbated owing to rising atmospheric CO₂, due to fossil fuel combustion and land-use practices, other human activities, and rising global temperatures. River fluxes of total reactive C, N, and P are projected to increase from late preindustrial time to the year 2100 by 150%, 380%, and 257%, respectively, modifying significantly the behavior of these element cycles in the coastal ocean, particularly in proximal environments. Despite the fact that the global shoal water carbonate mass has grown extensively since the LGM, the pH_T (pH values on the total proton scale) of global coastal waters has decreased from ~8.35 to ~8.18 and the carbonate ion concentration declined by ~19% from the LGM to late preindustrial time. The latter represents a rate of decline of about 0.028 μmol CO₃ ^2? per decade. In comparison, the decrease in coastal water pH_T from the year 1900 to 2000 was about 8.18–8.08 and is projected to decrease further from about 8.08 to 7.85 between 2000 and 2100, according to the IS92a business-as-usual scenario of CO₂ emissions. Over these 200 years, the carbonate ion concentration will fall by ~120 μmol kg^?1 or 6 μmol kg^?1 per decade. This decadal rate of decline of the carbonate ion concentration in the Anthropocene is 214 times the average rate of decline for the entire Holocene. Hence, when viewed against the millennial to several millennial timescale of geologic change in the coastal ocean marine carbon system, one can easily appreciate why ocean acidification is the “other CO₂ problem”.     

Tentative kinetic model for dolomite precipitation rate and its application to dolomite distribution

The dolomite problem has a long history and remains one of the most intensely studied and debated topics in geology. Major amounts of dolomite are not directly forming today from seawater. This observation has led many investigators to develop geochemical/hydrologic models for dolomite formation in diagenetic environments. A fundamental limitation of the current models for the growth of sedimentary dolomite is the dearth of kinetic information for this phase, in contrast to that available for calcite and aragonite.We present a simple kinetic model describing dolomite growth as a function of supersaturation using data from published high temperature synthesis experiments and our own experimental results. This model is similar in form to empirical models used to describe precipitation and dissolution rates of other carbonate minerals. Despite the considerable uncertainties and assumptions implicit in this approach, the model satisfies a basic expectation of classical precipitation theory, i.e., that the distance from equilibrium is a basic driving force for reaction rate. The calculated reaction order is high (~ 3), and the combined effect of high order and large activation energy produces a very strong dependence of the rate on temperature and the degree of supersaturation of aqueous solutions with respect to this phase.Using the calculated parameters, we applied the model to well-documented case studies of sabkha dolomite at Abu Dhabi (Persian Gulf), and organogenic dolomite from the Gulf of California. Growth rates calculated from the model agree with independent estimates of the age of these dolomites to well within an order of magnitude. A comparison of precipitation rates in seawater also shows the rate of dolomite precipitation to converge strongly with that of calcite with increasing temperature. If correct, this result implies that dolomite may respond to relatively modest warming of surface environments by substantial increases in accumulation rate, and suggests that the distribution of sedimentary dolomite in the rock record may be to some extent a temperature signal.     

The Role of CaCO<Subscript>3</Subscript> Reactions in the Contemporary Oceanic CO<Subscript>2</Subscript> Cycle

The present analysis adjusts previous estimates of global ocean CaCO₃ production rates substantially upward, to 133 × 10¹² mol yr^?1 plankton production and 42 × 10¹² mol yr^?1 shelf benthos production. The plankton adjustment is consistent with recent satellite-based estimates; the benthos adjustment includes primarily an upward adjustment of CaCO₃ production on so-called carbonate-poor sedimentary shelves and secondarily pays greater attention to high CaCO₃ mass (calcimass) and turnover of shelf communities on temperate and polar shelves. Estimated CaCO₃ sediment accumulation rates remain about the same as they have been for some years: ~20 × 10¹² mol yr^?1 on shelves and 11 × 10¹² mol yr^?1 in the deep ocean. The differences between production and accumulation of calcareous materials call for dissolution of ~22 × 10¹² mol yr^?1 (~50 %) of shelf benthonic carbonate production and 122 × 10¹² mol yr^?1 (>90 %) of planktonic production. Most CaCO₃ production, whether planktonic or benthonic, is assumed to take place in water depths of <100 m, while most dissolution is assumed to occur below this depth. The molar ratio of CO₂ release to CaCO₃ precipitation (CO₂↑/CaCO₃↓) is <1.0 and varies with depth. This ratio, Ψ, is presently about 0.66 in surface seawater and 0.85 in ocean waters deeper than about 1000 m. The net flux of CO₂ associated with CaCO₃ reactions in the global ocean in late preindustrial time is estimated to be an apparent influx from the atmosphere to the ocean, of +7 × 10¹² mol C yr^?1, at a time scale of 10²–10³ years. The CaCO₃-mediated influx of CO₂ is approximately offset by CO₂ release from organic C oxidation in the water column. Continuing ocean acidification will have effects on CaCO₃ and organic C metabolic responses to the oceanic inorganic C cycle, although those responses remain poorly quantified.