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A box model of the atmosphere and ocean was developed to investigate how geochemical distributions extant during the late Pleistocene may have come about. The model simulates the regional distribution of calcium carbonate dissolution as well as the chemical oceanography and atmospheric CO2, •5•3C, and radiocarbon. If the downward biological flux of particulate carbon increases by a factor of 2 to 3 in the Antarctic and if this increase is combined with a relative increase of the Atlantic sector Antarctic Bottom Water (AABW) versus North Atlantic Deep Water (NADW) source ratio from 1:3 to about 2:1, then the model predicts several changes that seem to be recorded in the sedimentary record, as follows: (1) A global redistribution of nutrients and •2C from the intermediate to deep water takes place with the Ariantic intermediate water phosphate decreasing 0.6 gmole kg -• and the •5•3C increasing 0.3 to 0.5 %0. (2) The dissolved oxygen level of the deep sea is reduced from an average of about 180 to 70 gmole kg -•, but the intermediate water oxygen declines only a small amount. (3) The decrease in intermediate water nutrient concentration results in lower average organic carbon and calcium carbonate production in the warm surface ocean. (4) The atmospheric CO2 decreases by 90 to 110 ppm. (5) Initially, a global increase in calcium carbonate dissolution occurs, which is followed by a relaxation toward better preservation than exists for the present ocean. In the model in this paper the reduction of NADW by itself does not produce these effects. Rather, the nutrient decrease that does occur is found mostly in North Pacific intermediate water, and the model atmospheric CO2 decrease is only 10 to 30 ppm. It is Copyfight 1988 by the American Geophysical Union. Paper number 8P0385. 0883-8305/88/008P-0385510.00 observed that 92% of the atmospheric CO2 change takes place according to a 200-year time constant in the model. This corresponds to the response time of the upper ocean and atmosphere to a change in the stationary state atmospheric Pco,. Thus, according to this model, the time lag between the nutrient-based cause and the atmospheric CO2 response is not expected to be particularly large. /41/4 Keir: Late Pleistocene Ocean Geochemistry postulating either a greater utilization of high-latitude preformed nutrients, or a constant polar production with a lower rate of vertical mixing [Knox and McElroy, 1984; Sarmiento and Toggweiler, 1984; Siegenthaler and Wenk, 1984]. It has also been suggested that increased low-latitude, upper ocean mixing may have contributed to a lower atmosphere CO2 [Boyle, 1986]. In only two of the previous studies has the response of the ocean calcium carbonate dissolution flux been considered explicitly [Keir and Berger, 1983; Ennever and McElroy, 1985], and in none of the previous work has the potential for regionally varying dissolution response been addressed from a model simulation standpoint. Because the ocean carbonate dissolution pattern has varied in regionally diverse ways during the Plei...

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[1] The effect of volcanic activity on submarine hydrothermal systems has been well documented along fast-and intermediate-spreading centers but not from slow-spreading ridges. Indeed, volcanic eruptions are expected to be rare on slow-spreading axes. Here we report the presence of hydrothermal venting associated with extremely fresh lava flows at an elevated, apparently magmatically robust segment center on the slow-spreading southern Mid-Atlantic Ridge near 5°S. Three high-temperature vent fields have been recognized so far over a strike length of less than 2 km with two fields venting phase-separated, vapor-type fluids. Exit temperatures at one of the fields reach up to 407°C, at conditions of the critical point of seawater, the highest temperatures ever recorded from the seafloor. Fluid and vent field characteristics show a large variability between the vent fields, a variation that is not expected within such a limited area. We conclude from mineralogical investigations of hydrothermal precipitates that vent-fluid compositions have evolved recently from relatively oxidizing to more reducing conditions, a shift that could also be related to renewed magmatic activity in the area. Current high exit temperatures, reducing conditions, low silica contents, and high hydrogen contents in the fluids of two vent sites are consistent with a shallow magmatic source, probably related to a young volcanic eruption event nearby, in which basaltic magma is actively crystallizing. This is the first reported evidence for direct magmatic-hydrothermal interaction on a slow-spreading mid-ocean ridge.Components: 8945 words, 9 figures, 3 tables.Keywords: mid-ocean ridge; eruption; vent fauna; fluids; phase separation.Index Terms: 1032 Geochemistry: Mid-oceanic ridge processes (3614, 8416); 0450 Biogeosciences: Hydrothermal systems (1034, 3017, 3616, 4832, 8135, 8424); 1034 Geochemistry: Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424). Haase, K. M., et al. (2007), Young volcanism and related hydrothermal activity at 5°S on the slow-spreading southern Mid-

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