Rare Earth Element patterns as indicators of trace metal sources to the Pacific Equatorial Undercurrent
Abstract
The Pacific Equatorial Undercurrent (EUC) has high Fe concentration and may represent the principle pathway for supply of this limiting micronutrient to the HNLC region of the eastern equatorial Pacific. Understanding this pathway is therefore important to assess past and future changes in the Pacific carbon cycle. The input of Fe to the EUC at its source, close to Papua New Guinea, is unclear and may come from rivers, sediments, or hydrothermal activity. Limited Rare Earth Elements (REE) data has previously suggested the potential of REE patterns to assess the source of metals in the EUC. In this study, we have fully explored this potential by measurement of about 250 REE patterns in the upper 300 m of the central and eastern Pacific, and in the Bismark Basin where the EUC is sourced. Full REE profiles were measured by HR-ICP-MS (Element2) with typical precisions of 2% and are compared with T, S, nutrient, ADCP, Fe, and Al measurements made on the same cruise (Biocomplexity 2005, RV Revelle). The EUC has a characteristic Ce concentration somewhat higher than surrounding Pacific waters and this concentration remains approximately constant with longitude at 2.5 pmol/kg. The high Ce values are observed particularly in the upper layer of the EUC with the core centred around 120 m at 140°W, shoaling to 100 m by 134°W. This excess Ce is possibly sourced by horizontal advection in the New Guinea Coastal Current and New Guinea Coastal Undercurrent (NGCU) which flow from the Coral Sea and form a large part in EUC. Relative to seawater of the Coral Sea, the EUC has higher REE concentrations, indicating the incorporation of metals during transit of these currents around Papua New Guinea. EUC REE patterns also exhibit well- developed MREE enrichment which probably reflects an island weathering signature as previously observed in the Sepik River and its estuary. Increase of MREE enrichment with depth indicates that the probable source is the shelf of Papua New Guinea, supplied in turn by a large fraction of the Sepik River sediment load. Our REE data support the idea that isopycnal transport into NGCU (and other western boundary equator-flowing undercurrents) may provides a source of dissolved and particulate Al, Fe, REE and other lithogenic elements to the EUC, which are ultimately transported east along the equator to the eastern Pacific.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFMOS33A0987Y
- Keywords:
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- 4800 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL (0460)