摘要:

The results of a 10‐year time series study of the chemistry of the Amazon River mainstem near Manaus, Brazil, are presented. All variables measured showed distinct seasonal patterns linked to the discharge hydrograph except respiration rate and PO4−3concentration. Stepwise multiple regression analysis showed that alkalinity, calcium, fine suspended sediment, and sulfate were correlated, primarily, with the percentage of the total water discharge that was derived from Andean drainages. Silicate, potassium, and the weight percentages of carbon and nitrogen in the coarse suspended sediment were correlated with the percentage of water attributable to local lowland drainages. These correlations suggest that seasonal cycles of these variables were controlled by their source strength. Coarse suspended sediment and the concentration all of the particulate carbon, nitrogen, and phosphorus species were correlated with river surface slope, suggesting that the seasonal cycles of these variables were controlled by river turbulence. The biogeochemically active elements O2, CO2, NO3−and SO4=, along with pH, Na, and Cl were all highly correlated with river discharge. The shapes of the seasonal cycles of O2and CO2and much of their amplitude could be reproduced by a quasi steady state model in which respiration was balanced by air‐water gas exchange. In the model, increases in river depth during the annual cycle result in increased depth‐integrated respiration rates. This produces a drawdown of O2concentration, which increases air‐water gas exchange, until the two processes are in balance. Thus the model produces seasonal cycles in which minimum dissolved O2and maximum dissolved CO2coincide with high water and the converse at low water, in agreement with the observations. The remainder of the amplitude signal was probably either advected in from upstream or produced by lateral exchange with the fringing

ISSN:0886-6236    

DOI:10.1029/95GB01145

年代:1995

数据来源: WILEY

摘要:

We used a process‐based model of ecosystem biogeochemistry (MBL‐GEM) to evaluate the effects of global change on carbon (C) storage in mature tropical forest ecosystems in the Amazon Basin of Brazil. We first derived a single parameterization of the model that was consistent with all the C stock and turnover data from three intensively studied sites within the Amazon Basin that differed in temperature, rainfall, and cloudiness. The range in temperature, soil moisture, and photosynthetically active radiation (PAR) among these sites is about as large as the anticipated changes in these variables in the tropics under CO2‐induced climate change. We then tested the parameterized model by predicting C stocks along a 2400‐km transect in the Amazon Basin. Comparison of predicted and measured vegetation and soil C stocks along this transect suggests that the model provides a reasonable approximation of how climatic and hydrologic factors regulate present‐day C stocks within the Amazon Basin. Finally, we used the model to predict and analyze changes in ecosystem C stocks under projected changes in atmospheric CO2and climate. The central hypothesis of this exercise is that changes in ecosystem C storage in response to climate and CO2will interact strongly with changes in other element cycles, particularly the nitrogen (N) and phosphorus (P) cycles. We conclude that C storage will increase in Amazonian forests as a result of (1) redistribution of nutrients from soil (with low C:nutrient ratios) to vegetation (with high C:nutrient ratios), (2) increases in the C:nutrient ratio of vegetation and soil, and (3) increased sequestration of external nutrient inputs by the ecosystem. Our analyses suggest that C:nutrient interactions will constrain increases in C storage to a maximum of 63 Mg/ha during the next 200 years, or about 16% above present‐day stocks. However, it is impossible to predict how much smaller the actual increase in C storage will be until more is known about the controls on soil P availability. On the basis of these analyses, we identify several topics for further research in the moist tropics that must be addressed to resolve these un

ISSN:0886-6236    

DOI:10.1029/95GB01736

年代:1995

数据来源: WILEY

摘要:

Measurements of dissolved methane in the surface waters of the western Sea of Okhotsk are evaluated in terms of methane exchange rates and are used to assess the magnitude of seasonal variations of methane fluxes from the ocean to the atmosphere in this area. Methane concentrations northeast of Sakhalin were observed to range from 385 nmol L−1under the ice cover in winter to 6 nmol L−1in the icefree midsummer season. The magnitude of supersaturations indicates that this part of the Okhotsk Sea is a significant source for atmospheric methane. From the seasonal variation of the supersaturations in the surface waters it is evident that the air‐sea exchange is interrupted during the winter and methane from sedimentary sources accumulates under the ice cover. According to our measurements an initial early summer methane pulse into the atmosphere of the order of 560 mol km−2d−1can be expected when the supersaturated surface waters are exposed by the retreating ice. The methane flux in July is approximately 150 mol km−2d−1which is of the order of the average annual flux in the survey area. The magnitude of the seasonal CH4flux variation northeast of Sakhalin corresponds to an amount of 7.3 × 105g km−2whereby 74% or 5.4 × 105g km−2are supplied to the atmosphere between April and July. For the whole Sea of Okhotsk the annual methane flux is roughly 0.13 × 1012g (terragrams), based on the assumption that 15% of the entire area emit methane. Variations of long‐term data of atmospheric methane which are recorded at the same latitude adjacent to areas with seasonal ice cover show a regional methane pulse between April and July. The large‐scale level of atmospheric methane in the northern hemisphere undergoes an amplitudinal variation of about 25 parts per billion by volume (ppbv) which translates into approximately 36 Tg. Thus the estimated 0.6 Tg of ice‐induced methane dynamics in northern latitudes can hardly explain this seasonal signal. However, the effects of seasonal ice cover on pulsed release of methane appear strong enough to contribute, in concert with other seasonal sources, to characteristic short‐term wobbles in the atmospheric methane budget which are ob

ISSN:0886-6236    

DOI:10.1029/95GB01144

年代:1995

数据来源: WILEY

摘要:

We estimate the global rate of biogenic silica production in the ocean to be between 200 and 280 × 1012mol Si yr−1. The upper limit is derived from information on the primary productivity of the oceans, the relative contribution of diatoms to primary production and diatom Si/C ratios. The lower limit is derived independently using a multi‐compartment model of nutrient transport and biogenic particle flux, and field data on the balance between silica production and dissolution in the upper ocean. Our upper limit is 30–50% lower than several previous estimates, due to new data indicating lower values for both the relative contribution of diatoms to primary productivity and their Si/C ratios. Globally, at least 50% of the silica produced by diatoms in the euphotic zone dissolves in the upper 100 m, resulting in an estimated export of 100–140 × 1012mol Si yr−lto the deep ocean. Our estimates correspond to a global mean rate of biogenic silica production between 0.6 and 0.8 mol Si m−2yr−1. Incubation experiments indicate that silica production rates exceed that mean by a factor of 3–12 in coastal areas and are 2–4 times less than the global average in the oligotrophic mid‐ocean gyres. The mean silica production rate in waters overlying diatomaceous sediments (approximately 10–12% of the surface area of the oceans) is 0.7–1.2 mol Si m−2yr−1. That rate is only slightly higher than the global average, indicating that the silica produced in those regions is only 10–25% of the global total. The estimated production of biogenic silica in surface waters of the mid‐ocean gyres is approximately equal to that for all major areas of opal sediment accumulation combined. Regional comparison of silica production and accumulation rates suggests a strongly bimodal character in the efficiency of opal preservation in the sea. In waters overlying diatom‐rich sediments 15–25% of the silica produced in the surface layer accumulates in the seabed, while virtually none of the silica produced in other areas is preserved. The global burial/production ratio of ˜ 3% is a composite of those two very different systems. The mechanisms leading to more efficient opal preservation in regions of silica accumulation are presently unknown, but they have no simple relationship to primary productivity. Regional differences in opal preservation appear to be controlled by factors such as low surface temperature, selective grazing and aggregate formation, which diminish the rate of silica dissolution in surface waters and/or ac

ISSN:0886-6236    

DOI:10.1029/95GB01070

年代:1995

数据来源: WILEY

摘要:

Two recent papers have suggested that there is an apparent imbalance in the global bomb14C budget. These results are based in part on calculations made using simple one‐dimensional ocean models. We recalculate the ocean inventory of bomb14C using a three‐dimensional ocean general circulation model, and find a significantly smaller inventory after about 1980 than in previous one‐dimensional model estimates. Our new estimate of the ocean inventory implies that the apparent imbalance in the global budget of bomb14C is smaller than previously thought and is not significantly different from zero. In addition, we use our three‐dimensional model to obtain whole ocean inventories of bomb14C during the Geochemical Ocean Sections Study (GEOSECS) period; these inventories can be used as benchmarks in calibrating one‐dimensional oce

ISSN:0886-6236    

DOI:10.1029/95GB01119

年代:1995

数据来源: WILEY

摘要:

N. Shackleton (1977) first proposed that changes in the marine δ13C record (Δδ13C) could be used to infer ice age changes in carbon storage on land. The previously published best estimate from the marine record is equivalent to about 490 Gt (0.32 Δδ13C). However, Adams et al. (1990) utilized a pollen database to estimate a 1350 Gt change in carbon storage, which would cause a Δδ13C of about 0.90‰. The nearly trillion ton difference in estimates amounts to almost half of the total carbon stored on land. To address the nature of this discrepancy, I have reexamined the terrestrial carbon record based on a new pollen database compiled by R. Webb and the Cooperative Holocene Mapping Project (COHMAP) group. I estimate about 750–1050 Gt glacial‐interglacial change in terrestrial carbon storage, with the range reflecting uncertainties in carbon storage values for different biomes. Additional uncertainties apply to rainforest and wetland extent and presence of C4 plants, which have a significantly different isotopic signature than C3 plants. Although some scenarios overlap a new estimate of carbon storage based on the oceanic Δδ13C record (revised to 0.40‰ and 610 Gt), most estimates seem to fall outside the envelope of uncertainty in the marine record. Several possible explanations for this gap involve: (1) a missing sink may be involved in land‐sea carbon exchange (e.g., continental slopes); (2) the geochemistry of the exchange process is not understood; (3) carbon storage by biome may have changed under ice age boundary conditions; or (4) the standard interpretation of whole ocean changes in the marine δ13C record requires reevaluation. This latter conclusion receives some support from comparison of the δ13C records for δ18O Stages 2 and 6. For the Stage 6 glacial, the δ13C changes are 50–60% larger than for the Stage 2 glacial. Yet implications of increased aridity are not supported by longterm trends in atmospheric dust loading. To summarize, the above analysis implies that, despite the uncertainties remaining in estimates of terrestrial carbon storage changes, a case can be made that our understanding of the transfer process is incomplete and that the eventual explanation may require clarification of factors affectin

ISSN:0886-6236    

DOI:10.1029/95GB01107

年代:1995

数据来源: WILEY

摘要:

Here we present results from a new method for detecting changes in atmospheric CO2based on δ13C analyses of selected peat components from a peat core in southern South America. The paleo‐CO2‐record has decadal resolution spanning the last 14,000 radiocarbon years and compares well with Antarctic ice core CO2data. Sharp peaks in CO2are detected during the late glacial, specifically at 10,200, 11,600, and 12,900 years B.P. When compared to two deep‐sea records interpreted to reflect changes in thermohaline circulation, these CO2pulses appear to relate to degassing events of the ocean associated with reinitiation of the thermohaline circulation occurring at this time. Concomitant decreases in atmospheric Δ14C during the late glacial are also consistent with a deep oceanic carbon source for the large atmospheric pulses in CO2. Results from a simple three‐box model indicate that the magnitude of the CO2released during these degassing events is compatible with our observations. The rate of degassing, however, is much slower than that observed in the paleo‐CO2‐record. Two broad Holocene CO2excursions are also identified with peaks at 4,200 and 7,700 years B.P. The driving mechanism behind these excursions appears to be different than those in the

ISSN:0886-6236    

DOI:10.1029/95GB01458

年代:1995

数据来源: WILEY