Fluvial discharge of silicate to the oceans: a global perspective

Publication Type Conference Proceedings
Year 1999
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Primary Author Milliman, John D.
Author Farnsworth, Katherine L.
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URL http://data.ecology.su.se/scopesi/fluvdisch.htm
DOI
Abstract Over the past 20 years we have accumulated a database for more than 1500 rivers, which collectively represent more than 85 percent of the total land area draining into the oceans. As such, this unique database affords us a unique opportunity to quantify and assess fluvial fluxes to the oceans from both regional and global perspectives. At present rivers discharge approximately 35,000 km3 of freshwater to the global ocean; another 3000 km3/yr are assumed to be contained in reservoirs behind dams. Subtracting this total from meteorologic runoff (precipitation minus evaporation: 39,700 km3/yr), groundwater flux to the global ocean would be 2000 km3/yr. By virtue of high meteorologic runoff, monsoonal climate, and large cumulative drainage area, the rivers draining southeast Asia and the high-standing islands of Oceania (including New Zealand, Indonesia and the Philippines) collectively contribute about 30% of the global freshwater flux, with northeastern South America (primarily the Amazon and Orinoco rivers) discharging another 25%. Suspended sediment discharge to the oceans is much more difficult to estimate, since the sediment load for any river depends on a number of highly variable factors, such as basin area, topography, climate, geology, landuse, etc. Tropical and sub-tropical rivers are particularly susceptible to high rates of erosion due since they drain high-standing, geologically young mountains, are influenced by a monsoonal climate, and their often small basin areas mean that they are more responsive to periodic events and that relatively little sediment is stored along their river courses. A first-order estimate indicates that rivers draining southern Asia and Oceania account for at least 75% of the sediment discharged annually to the oceans (estimated to be about 18.6 x 109 t/yr). Climate and the geological framework of the drainage basin also play important roles in determining dissolved sediment flux. While south Asia accounts for about 35 percent of the 3.9 x 109 t/yr discharged annually, Europe and eastern North America rivers collectively play much greater global roles than they do for either water or suspended solid discharge, contributing about 25 percent of the global dissolved flux. Regional differences in global fluxes of various dissolved species are particularly interesting. For instance, utilizing the extensive data base by Meybeck and Ragu (1996), the major fluvial source for Cl-1 flux to the global ocean is the rivers draining northern and southern Europe, with south Asian, eastern North American, the Eurasian Arctic and west African rivers also having relatively large inputs. We assume that this large Cl- flux mostly reflects drainage from large Paleozoic evaporitic sequences, but at least some of these elevated levels (the Weser River, for instance, has a salinity of about 2 psu!) may be anthropogenically induced. In contrast, the flux of dissolved silica to the global ocean reflects regional variations in SiO2 concentrations, which are largely controlled by climate and the nature of the eroded substrate. High-latitude rivers generally have low to very low SiO2 concentrations (1-3 ppm) compared to tropical rivers, which locally display concentrations greater than 30 ppm. As a result, south Asian rivers discharge about 40 percent of the 330 x 106 t/yr dissolved silica to the global ocean, and northeastern South American rivers account for another 20 percent. In contrast, Arctic rivers contribute only about 5 percent of the global total. One problem in assessing the significance of regional and global trends is that river discharge continually changes in response to both natural and anthropogenic perturbations: long-term variation in rainfall patterns, short-term El-Nino-induced droughts or floods, and human-engineered changes in river flow and drainage patterns. At present nearly all temperate and many high-latitude rivers have been totally or partially dammed or diverted, but many smaller tropical rivers in southeastern Asia and northern South America remain relatively free-flowing. On the other hand, hundreds (and often thousands) of years of poor agricultural practices have resulted in elevated suspended and dissolved sediment loads for south Asian and Oceania rivers, and accelerated deforestation in these regions undoubtedly will lead to even greater short-term fluxes. Given the obvious importance of low-latitude rivers in global fluxes, it is particularly essential that we document and understand these rivers as well as predicting what future changes may bring to them, the seas into which they flow, and perhaps the global ocean.
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Conference International Workshop on the Global Silica Cycle, Linköping, Sweden, October 3-5, 1999
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Notes This reference is an abstract only. The conference resulted in an eventual book publication: The SILICON CYCLE: Human Perturbations and Impacts on Aquatic Systems (SCOPE 66) ISBN: 1-59726-115-7 Island Press However, the paper presented does not appear to have resulted in its own chapter. As of July 6, 2007, inclusion of this matarial in printed or reviewed form has not been verified. The Stanford Library DOES have a copy of SCOPE 66 in Falconer... Call No: QH344 .S55 2006