Highlights of Recent GEOTRACES Field Programs

GEOTRACES is an international program for measuring key trace elements and isotopes in the oceans. The novel ultraclean all-titanium frame with 24 pristine PVDF samplers deployed from a kevlar hydrowire allows rapid and ultraclean sampling of deep ocean waters. First ever deep ocean sections of bio-limiting iron, manganese, and bio-interacting aluminium were obtained in the Arctic (2007), Atlantic (2010-2011) and Antarctic (2008) Oceans. The ~1800 km long section in the central Arctic Ocean shows elevated Fe and Mn in upper waters where the TransPolar Drift brings these metals from coastal Eurasian shelf and river sources. Deep waters are low in Fe and Mn except for a distinct hydrothermal plume over Gakkel Ridge.  In contrast the ~3000 km long section along Greenwich meridian in the Antarctic Ocean has very low dissolved Fe and Mn in surface waters. First ever evidence for Mn co-limitation with Fe and light limitation of phytoplankton growth in the Southern Ocean is supported by Mn covariances with 234Thorium and phosphate in upper waters. Towards the continental ice-sheet extruding from the Antarctic continent the dissolved Fe and Mn are extremely low. Low Fe and Mn is also found in transects across Weddell Sea and Drake Passage, yet with inputs from the shelves at both sides of the Antarctic Peninsula. The ~13000 km long West Atlantic section (65N to 50S) is the longest deep ocean transect of Fe, Mn, Al and other trace elements thus far. Multiple sources are apparent from aeolian dust, Amazon and Orinoco rivers, equatorial oxygen minimum zones, hydrothermal vents and frontal systems.

            In the Arctic Ocean and subArctic Gyre the vertical profiles of dissolved Al show strong covariance with silicate ascribed to incorporation of Al in opal of diatoms. However along the 13000 km West Atlantic transect the distribution of Al is the inverse of silicate, due to deep water scavenging of Al along the trajectory of southward flowing North Atlantic Deep Water as confirmed in ocean simulation modeling. Within the Antarctic Ocean the Al levels are extremely low and no longer related with the high dissolved silicate of the Southern Ocean.

            Near Bermuda excellent agreement was found for trace metal elements at the first-ever Geotraces crossover station between the US 2008 calibration station and the Netherlands 2010 section.

            First evidence of biological fractionation of stable isotopes of cadmium in the Southern Ocean is consistent with the previously reported biochemical function of Cd in carbonic anhydrase enzyme of certain diatoms.