Water mass analysis for the U.S. GEOTRACES (GA03) North Atlantic sections

Abstract

a b s t r a c tWe present the distributions of hydrographic properties (potential temperature, salinity, dissolved oxygen, and micromolar level inorganic macronutrients) along two sections occupied in the subtropical North Atlantic as part of the first U.S. GEOTRACES (GA03) survey during 2010 and 2011. The purpose of this work is to place subsequent papers in this special issue in a general context and to provide a framework in which the observed distributions of Trace Elements and Isotopes can be interpreted. Using these hydrographic properties we use a modified Optimum Multiparameter water mass analysis method to diagnose the relative contributions of various water types along the sections and rationalize their distributions. The water mass compositions appear largely consistent with what is understood from previous studies about the large scale circulation and ventilation of the North Atlantic, with perhaps one exception. We found that the North Atlantic Deep water both east and west of the Mid Atlantic Ridge is more strongly influenced by Iceland Scotland Overflow Water relative to Denmark Straits Overflow Water (about 3:1) than inferred from other tracer studies (typically 2:1). It remains unclear whether this is an artifact of our calculation or a real change in deep water composition in the decades between the determinations.

Coastal sources, sinks and strong organic complexation of dissolved cobalt within the US North Atlantic GEOTRACES transect GA03

“…In surface waters, this difference is explained by the fast movement of surface waters and timescales of the processes affecting cobalt cycling in this highly biologically active part of the water column. In mesopelagic waters (particularly between 1500 and 2000 m), differences were observed within a water mass characterized as > 55 % Upper Circumpolar Deepwater (UCPDW) via optimum multi-parameter analysis (OMPA; Jenkins et al, 2015), indicating some temporal variability at these depths even within the relatively short timescale between samplings.…”

The deep distributions of helium isotopes, radiocarbon, and noble gases along the U.S. GEOTRACES East Pacific Zonal Transect (GP16)

“…We adopt an empirical relationship between deep water ∆Ne and ∆He in an area without significant hydrothermal helium contributions, namely the North Atlantic away from Mid Atlantic Ridge. Using recent data from the U.S. North Atlantic GEOTRACES GA03 cruise (Jenkins et al, 2014a;Jenkins et al, 2014b) we find a molar ratio of 0.22 ± 0.01 between excess He and excess Ne below 1000 m depth. The fact that this value is significantly lower than the atmospheric abundance ratio of 0.288 is a reflection of the intertwined effects of thermal disequilibrium (Hamme and Severinghaus, 2007) and partial vs. complete bubble injection processes (Jenkins, 1988;Stanley et al, 2009b;Liang et al, 2013) during water mass formation.…”

The <sup>226</sup>Ra-Ba relationship in the North Atlantic during GEOTRACES-GA01

“…The PIW is episodically injected into the Irminger Sea and originates from either the Arctic Ocean or the Greenland shelf (Falina et al, 2012;Jenkins et al, 2015;Rudels et al, 2002).…”