Assessment of island arc contribution to global oceanic osmium budget
Abstract
Here, we investigate whether chemical weathering on island arcs provides a relatively large amount of mantle-derived Os to the oceans. Presently, 70-80% of Os dissolved in the oceans is estimated to come from continental sources; the remainder is derived from the mantle/extraterrestrial matter. Therefore, a record of the temporal evolution of seawater Os isotopes, potentially yields quantitative insights into past variations in terrestrial chemical weathering. Before this relationship can be precisely determined, however, further investigation into the parameters that control seawater Os isotopic composition is needed. Inputs to the global seawater Os budget from continental weathering, oceanic hydrothermal systems, and cosmic dust/meteorites have all been investigated. To date, a source with low 187Os/188Os input that is large enough to balance the high 187Os/188Os continental contribution has not been identified (Sharma et al., 2007). Island arcs have relatively large areas of mantle-derived rocks; therefore, they offer a possible low 187Os/188Os input missing in the global Os budget. Previous calculations of global Os fluxes coming from rivers have neglected inputs from island arcs due to their relatively small areal extent and low overall run-off. Recently, this assumption has been called into question based upon research suggesting that hydrothermal activity associated with island arc volcanism enhances chemical weathering, providing disproportionately larger amounts of dissolved cations to the oceans (Allegre et al., 2010; Rad et al., 2007). Moreover, investigations have indicated that rivers draining high standing island arc volcanoes (headwaters > 1,000 m) provide some of the worlds largest fluxes of sediments and cations to the marine environment (Goldsmith et al., 2008; Lyons et al., 2005). The north island of New Zealand is well suited for assessing the contribution of island arc systems to Os global budget. We collected waters from 45 rivers, 8 springs, and 3 precipitation events, as well as river bed sediments from 43 rivers around the perimeter of the island. Initial N-TIMS osmium isotope data from 2 rivers, 4 springs and 1 precipitation event show a spread of 187Os/188Os from 0.27-0.57. The [Os] range from 1-14 pg/kg. These results suggest that water draining the island is nonradiogenic and that New Zealand and other island arc systems may indeed represent an important component of global Os cycle.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V23B2421B
- Keywords:
-
- 1000 GEOCHEMISTRY;
- 1039 GEOCHEMISTRY / Alteration and weathering processes;
- 1040 GEOCHEMISTRY / Radiogenic isotope geochemistry