Trace metal distributions in the N Atlantic: What do we know, what should we be looking for? Chris Measures University of Hawaii

To identify processes and quantify fluxes that control the distributions of key trace elements and isotopes in the ocean, and to establish the sensitivity of these distributions to changing environmental conditions Guiding Mission

GEOTRACES Science Plan identified processes Investigating processes not limited to any one basin

Fluxes and processes for GEOTRACES in the North Atlantic Input pathways: Rivers Atmosphere Hydrothermal Inflows from other basins Redistribution processes Physical Biological Fractionation processes along distribution paths

Tracers of ventilation oceanic redox state Tracers of circulation: Paleo proxies for AMOC Mode water formation Mediterranean outflow Penetration of anthropogenic signals into the oceans

Inputs: Rivers Shelf sediments Atmospheric deposition Extra-basin ocean water fluxes: Mediterranean, Nordic Seas, AABW

Arctic Transect Antarctic Transect UK D UK/D/Es USA F F/Can Nl UK USA/D/Mex F/Es/It The Atlantic Basin workshop in Oxford, Sept 2007 identified potential tracks and sponsors

So, how shall we do this? Where shall we sample, and why?

We know most about Atlantic TM systematics from parameters that have long time series or for which we have extensive spatial distributions What do we already know about TEI in the Atlantic? The anthropogenic Pb data is particularly valuable since it combines a relatively long time-series from the BATS site as well as extensive sections

Anthropogenic tracers have the added value that they allow us to investigate propagation of the signal through the system as the background that they penetrate is low or non existent e.g. CFCs, bomb H 3 are extremely valuable but are limited by the time since production of the tracer/event Pb has the added advantage of multiple isotopes that can be used to fingerprint sources and to illuminate the penetration process There is also a paleo record in corals that helps extend the record

Data from Ed Boyle And others

Pb isotope profiles near Bermuda Data from Ed Boyle And others

Pb isotopes are particularly valuable at showing the propagation of the Pb signal into the interior of the ocean BATS site Eastern Atlantic section

Data from Ed Boyle

Isotope records from corals can be used to reconstruct oceanic surface water concentrations Data from Ed Boyle

Pettaquamscutt Estuary, Rhode Island, USA (near URI Graduate School of Oceanography): a permanently anoxic estuary with varved sediments (established by 137 Cs, 14 C, and 210 Pb: Lima, King, et al., Geochim. Cosmochim. Acta.) Anthropogenic Lead deposition in the Northeast U.S. (past 250 years) Data from Ed Boyle

Southern Preserve Coral 206Pb/207Pb, Pb/Ca Data from Ed Boyle

All periods, scaled to maximum Pb Data from Ed Boyle

Aluminium is also a valuable tracer No isotopes or transient tracer properties But is a major component of continental crust So can trace flux of crustal materials with the ocean

Mineral deposition to the surface ocean g m -2 yr -1 (Duce et al., 1991) The N Atlantic has one of the highest dust fluxes in the world atmospheric deposition is an important vector for many TEI But oceanic deposition maps are ground-truthed from land-based stations

Calculated dust deposition (from surface Al) agrees well with GESAMP predictions--but some differences additional maxima at 30˚N Measures et al., 2008 Global Biogeochemical Cycles

Penetration of this surface signal into STMW is physical, not biological Mediterranean also has a large atmospheric signal Measures et al., 2008 Global Biogeochemical Cycles

Large lateral gradients in dust deposition across the Atlantic

Hanawa and Talley (2001) show two subtropical mode water types: Classic 18˚ water (Worthington, 1959) and Madeira Mode water (Siedler et al, 1987)

Understanding the systematics of Fe input and cycling is a crucial goal of GEOTRACES

Only 5 full depth intercalibrated Fe intercalibrated Fe profiles in the N profiles in the N Atlantic Atlantic Data from Ed Boyle

Large gradients in atmospheric deposition shows importance of atmospheric input for Fe. Low atmospheric deposition leads to potential Fe limitation in the N Atlantic Measures et al., 2008 Global Biogeochemical Cycles

Variations in atmospheric transport and deposition combine with stratification to vary surface water signals by trapping surface inputs in a shallow layer Sedwick, unpublished data Figure shows Fe profiles from BATS site displaying time variation of surface water concentrations

Significant Fe maximum in the subsurface waters of the equatorial Atlantic Data from Ed Boyle

High resolution, but shallow CLIVAR sampling confirms the association of elevated Fe with the oxygen minimum Measures et al., 2008 Global Biogeochemical Cycles

At Chl maximum see depletion in profiles of Fe, active biological uptake Under the Saharan dust plume Measures et al., 2008 Global Biogeochemical Cycles

Atlantic Fe Fe (nmol/kg) CLIVAR sampling in context of whole Atlantic nitrate data Preliminary results from CLIVAR cruises show comparable richness of features in Fe distribution. Gideon Henderson

1˚ 4˚ 8˚ Sampling resolution As we increase sampling density, resolution of features improves Fe Measures, Hatta unpublished data

Montoya/Zehr 2006 Seward Johnson Data from Mak Saito Co data section from Mak Saito Showing micronutrient like vertical distribution with Amazon input in the western basin surface waters

Top: Cobalt time series at BATS (BTM) in 1999 from ~40m depth collected by MITESS Bottom: Temperature data from 15m and 35m on the same Bermuda Testbed Mooring Saito and Moffett, Geochim. Cosmochim. Acta 2002 Data from Mak Saito

Figure from Ken Bruland showing Zn contamination problems and their resolution from sacrificial anodes on a rosette

Acknowledgements The US CLIVAR community who have supported our TM program’s shipboard program NSF OCE for funding the CLIVAR cruises will change the way we see the oceans Ed Boyle, Peter Sedwick, Mak Saito, Ken Bruland