Strong Input and Removal of Rare Earth Elements (REEs) Affect Dissolved Nd Isotope Composition of Seawater in the Panama Basin and the Eastern Equatorial Pacific

Abstract:

The distributions of dissolved REEs and Nd isotopes are controlled by inputs from land and water mass mixing. We present new data from the Panama Basin in the Eastern Equatorial Pacific (EEP) extending previous studies in the frame of the German SFB 754 project. The samples were taken following GEOTRACES protocols along a section from close to the Panamanian coast into the deep Panama Basin during FS Meteor cruise M90 in October/November 2012.

Elevated Nd concentrations near 13 pmol/kg are found at the surface, which rapidly decrease to a subsurface minimum of 8 pmol/kg near 100m depth and then increase with water depth reaching maximum values of 18 pmol/kg at 3000 m water depth. However, these deep water concentrations are more than a factor of 2 lower than observed for North Pacific Deep Water (NPDW), which is the prevailing Pacific deep water mass at the sampling locations. All the REEs are depleted compared to NPDW suggesting that efficient uptake and scavenging dominate compared to release from remineralized particles.

The surface waters show the most radiogenic Nd isotope values (εNd = +4.3) so far obtained globally. In combination with the Nd concentration maxima at the surface this suggests riverine dissolved and fine grained particulate inputs from southern Panama and Colombia where highly radiogenic volcanic rocks are exposed. Elevated εNd values above -1 in the entire water column are more radiogenic than in NPDW in the Central Pacific and in waters further south in the EEP, which confirms that release of REEs from the sinking volcanogenic material affects the entire water column.

These data clearly document that significant inputs from land combined with efficient scavenging and removal in surface and deep waters control the distribution of REEs and Nd isotopes in the Panama Basin and the adjacent EEP (Grasse et al., 2012), which also has important implications for the distribution of other trace metals.