1,721,027 research outputs found
Neogene foreland evolution of the southern central Andes and its relationship to ancient strain history and varying climates - an integrated approach
No full textDistinct roles of the Southern Ocean and North Atlantic in the deglacial atmospheric radiocarbon decline
No full textIn the context of the atmospheric CO214C/C (?Catm14) changes since the last ice age, two episodes of sharp ?Catm14 decline have been related to either the venting of deeply sequestered low-14C CO2 through the Southern Ocean surface or the abrupt onset of North Atlantic Deep Water (NADW) formation. In model simulations using an improved reconstruction of 14C production, Atlantic circulation change and Southern Ocean CO2 release both contribute to the overall deglacial ?Catm14 decline, but only the onset of NADW can reproduce the sharp ?Catm14 declines. To fully simulate ?Catm14 data requires an additional process that immediately precedes the onsets of NADW. We hypothesize that these “early” ?Catm14 declines record the thickening of the ocean's thermocline in response to reconstructed transient shutdown of NADW and/or changes in the southern hemisphere westerly winds. Such thermocline thickening may have played a role in triggering the NADW onsets
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The effects of secular calcium and magnesium concentration changes on the thermodynamics of seawater acid/base chemistry: Implications for Eocene and Cretaceous ocean carbon chemistry and buffering
No full textReconstructed changes in seawater calcium and magnesium concentration ([Ca2+], [Mg2+]) predictably affect the ocean’s acid/base and carbon chemistry. Yet inaccurate formulations of chemical equilibrium “constants” are currently in use to account for these changes. Here we develop an efficient implementation of the MIAMI Ionic InteractionModel to predict all chemical equilibrium constants required for carbon chemistry calculations under variable [Ca2+] and [Mg2+]. We investigate the impact of [Ca2+] and [Mg2+] on the relationships among the ocean’s pH,CO2, dissolved inorganic carbon (DIC), saturation state of CaCO3 (?), and buffer capacity. Increasing [Ca2+] and/or [Mg2+] enhances “ion pairing,” which increases seawater buffering by increasing the concentration ratio of total to “free” (uncomplexed) carbonate ion. An increase in [Ca2+], however, also causes a decline in carbonate ion to maintain a given ?, thereby overwhelming the ion pairing effect and decreasing seawater buffering. Given the reconstructions of Eocene [Ca2+] and [Mg2+] ([Ca2+] ~20mM; [Mg2+]~30mM), Eocene seawater would have required essentially the same DIC as today to simultaneously explain a similar-to-modern ? and the estimated Eocene atmospheric CO2 of ~1000 ppm. During the Cretaceous, at ~4 times modern [Ca2+], ocean buffering would have been at a minimum. Overall, during times of high seawater [Ca2+], CaCO3 saturation, pH, and atmospheric CO2 were more susceptible to perturbations of the global carbon cycle. For example, given both Eocene and Cretaceous seawater [Ca2+] and [Mg2+], a doubling of atmospheric CO2 would require less carbon addition to the ocean/atmosphere system than under modern seawater composition. Moreover, increasing seawater buffering since the Cretaceous may have been a driver of evolution by raising energetic demands of biologically controlled calcification and CO2 concentration mechanisms that aid photosynthesis
Shortcomings of the isolated abyssal reservoir model for deglacial radiocarbon changes in the mid-depth Indo-Pacific Ocean
No full textSeverely negative ?14C anomalies from the mid-depth Pacific and the Arabian Sea have been taken as support for the hypothesized deglacial release of a previously isolated, extremely 14C-deplete deep ocean carbon reservoir. We report box model simulations that cast doubt on both the existence of the hypothesized deep reservoir and its ability to explain the mid-depth ?14C anomalies. First, the degree of ice age isolation needed to substantially reduce the deep ?14C of the deep reservoir causes anoxia and the trapping of alkalinity from CaCO3 dissolution, the latter increasing atmospheric CO2. Second, even with a completely 14C-free deep reservoir, achieving the mid-depth ?14C anomalies of observed duration requires ad hoc stifling of aspects of deep circulation to prevent rapid dissipation of the anomalous 14C-free carbon to the rest of the ocean and the atmosphere. We suggest that the mid-depth anomalies do not record basin-scale ?14C changes but are instead local phenomena
Carbon dioxide effects of Antarctic stratification, North Atlantic Intermediate Water formation, and subantarctic nutrient drawdown during the last ice age: Diagnosis and synthesis in a geochemical box model
No full textIn a box model synthesis of Southern Ocean and North Atlantic mechanisms for lowering CO2 during ice ages, the CO2 changes are parsed into their component geochemical causes, including the soft-tissue pump, the carbonate pump, and whole ocean alkalinity. When the mechanisms are applied together, their interactions greatly modify the net CO2 change. Combining the Antarctic mechanisms (stratification, nutrient drawdown, and sea ice cover) within bounds set by observations decreases CO2 by no more than 36 ppm, a drawdown that could be caused by any one of these mechanisms in isolation. However, these Antarctic changes reverse the CO2 effect of the observed ice age shoaling of North Atlantic overturning: in isolation, the shoaling raises CO2 by 16 ppm, but alongside the Antarctic changes, it lowers CO2 by an additional 13 ppm, a 29 ppm synergy. The total CO2 decrease does not reach 80 ppm, partly because Antarctic stratification, Antarctic sea ice cover, and the shoaling of North Atlantic overturning all strengthen the sequestration of alkalinity in the deepest ocean, which increases CO2 both by itself and by decreasing whole ocean alkalinity. Increased nutrient consumption in the sub-Antarctic causes as much as an additional 35 ppm CO2 decrease, interacting minimally with the other changes. With its inclusion, the lowest ice age CO2 levels are within reach. These findings may bear on the two-stepped CO2 decrease of the last ice age
The polar ocean and glacial cycles in atmospheric CO2 concentration
No full textGlobal climate and the atmospheric partial pressure of carbon dioxide ( ) are correlated over recent glacial cycles, with lower during ice ages, but the causes of the changes are unknown. The modern Southern Ocean releases deeply sequestered CO2 to the atmosphere. Growing evidence suggests that the Southern Ocean CO2 ‘leak’ was stemmed during ice ages, increasing ocean CO2 storage. Such a change would also have made the global ocean more alkaline, driving additional ocean CO2 uptake. This explanation for lower ice-age , if correct, has much to teach us about the controls on current ocean processes
Sinking organic matter spreads the nitrogen isotope signal of pelagic denitrification in the North Pacific
No full textCulture studies of denitrifying bacteria predict that denitrification will generate equivalent gradients in the ?15N and ?18O of deep ocean nitrate. A depth profile of nitrate isotopes from the Hawaii Ocean Time-series Station ALOHA shows less of an increase in ?18O than in ?15N as one ascends from abyssal waters into the denitrification-impacted mid-depth waters. A box model of the ocean nitrate N and O isotopes indicates that this is the effect of the low latitude nitrate assimilation/regeneration cycle: organic N sinking out of the surface spreads the high-?15N signal of pelagic denitrification into waters well below and beyond the suboxic zone, whereas the nitrate ?18O signal of denitrification can only be transmitted by circulation in the interior
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North Atlantic ventilation of “southern-sourced” deep water in the glacial ocean
No full textOne potential mechanism for lowering atmospheric CO2 during glacial times is an increase in the fraction of the global ocean ventilated by the North Atlantic, which produces deep water with a low concentration of unused nutrients and thus drives the ocean's biological pump to a high efficiency. However, the data indicate that during glacial times, a water mass low in 13C/12C and 14C/C occupied the deep Atlantic, apparently at the expense of North Atlantic Deep Water (NADW). This water is commonly referred to as “southern-sourced” because of its apparent entry into the Atlantic basin from the South, prompting the inference that it was ventilated at the Southern Ocean surface. Here, we propose that this deep Atlantic water mass actually included a large fraction of North Atlantic-ventilated water, the chemical characteristics of which were altered by recirculation in the deep Southern and Indo-Pacific oceans. In an ocean model sensitivity experiment that reduces Antarctic Bottom Water formation and weakens its overturning circulation, we find that a much greater fraction of NADW is transported into the Southern Ocean without contacting the surface and is entrained and mixed into the southern-sourced deep water that spreads into the global abyssal ocean. Thus, North Atlantic ventilation takes over more of the ocean interior, lowering atmospheric CO2, and yet the abyssal Atlantic is filled from the South with old water low in 13C/12C and 14C/C, consistent with glacial data
Analysis of global surface ocean alkalinity to determine controlling processes
No full textThe export of calcium carbonate (CaCO3) from the surface ocean is poorly constrained. A better understanding of the magnitude and spatial distribution of this flux would improve our knowledge of the ocean carbon cycle and marine biogeochemistry. Here, we investigate controls over the spatial distribution of total alkalinity in the surface global ocean and produce a tracer for CaCO3 cycling. We took surface ocean bottle data for total alkalinity from global databases (GLODAP, CARINA, PACIFICA) and subtracted the effects of several processes: evaporation and precipitation, river discharge, and nutrient uptake and remineralization. The remaining variation in alkalinity exhibits a robust and coherent pattern including features of large amplitude and spatial extent. Most notably, the residual variation in alkalinity is more or less constant across low latitudes of the global ocean but shows a strong polewards increase. There are differences of ~ 110 ?mol kg- 1 and ~ 85 ?mol kg- 1 between low latitudes and the Southern Ocean and the subarctic North Pacific, respectively, but, in contrast, little increase in the high-latitude North Atlantic. This global pattern is most likely due to production and export of CaCO3 and to physical resupply of alkalinity from deep waters. The use of river corrections highlights the large errors that are produced, particularly in the Bay of Bengal and the North Atlantic, if alkalinity normalization assumes all low salinities to be caused by rainfall. The residual alkalinity data can be used as a tracer to indicate where in the world’s ocean most CaCO3 export from the surface layer takes place, and of future changes in calcification, for instance due to ocean acidification
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Response to Comment by Zeebe and Tyrrell on “The effects of secular calcium and magnesium concentration changes on the thermodynamics of seawater acid/base chemistry: Implications for the Eocene and Cretaceous Ocean carbon chemistry and buffering”
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