PMEL Earth-Ocean Interactions Program logo National Oceanic and Atmospheric Administration Pacific Marine Environmental Laboratory Earth-Ocean Interactions Program

 

Joseph Resing

Research Project:
Climate Variability and Predictability (CLIVAR)
CLIVAR project website: www.clivar.org

  CLIVAR survey lines
Figure 1. U.S. Repeat Hydrography cruises. Samples have been taken from cruises A16N, P2, P16S and P16N.  I8S, I9N, I6S and either I7N or S4P will be sampled in the future. (click for full size)

 

Introduction
Atmospheric carbon dioxide concentrations affect the earth’s radiation balance which is a defining component of planetary equilibrium temperature and climate. Modeling the redistribution of atmospheric carbon dioxide between atmospheric, oceanic and crustal reservoirs is critical for understanding climate response to current and future anthropogenic carbon dioxide releases and is paramount for accurate prognostic climate models. Transfer of carbon dioxide across the atmospheric-oceanic boundary consists of two main components. First , there is a physical component where dissolution of carbon dioxide is moved into the ocean interior via mixing and entrainment. Second, is a biological component, called the ‘biological pump,’ where primary production within the euphotic zone fixes carbon dioxide gas into organic carbon particles which are then exported to the deep ocean and sediments. In the latter system there is a mounting body of evidence that the rate of transfer of carbon from the surface ocean to the crust is moderated by concentrations of biologically-available Fe. Fe is an important element for photosynthesis but is also a trace element in surface seawater and as a result controls the rate photosynthetic organisms fix carbon dioxide in large High Nutrient, Low Chlorophyll regions in the ocean.

A16N CLIVAR transect line  
Figure 2. Meridional Atlantic transect along 20° W from the A16N CLIVAR cruise in 2003.  

Thus, to better understand how the global biogeochemical ocean carbon cycle operates it is important to investigate and quantify all transport processes which deliver Fe and other trace nutrients into the surface ocean as well as the factors controlling them. This is particularly true for atmospheric transport and dissolution of continentally derived mineral dust to surface ocean areas located great distances from continental land masses. While other global ocean studies such as WOCE and JGOFS have quantified the global distributions of temperature, salinity, nutrients and a number of other tracers there is a comparatively small pool of data for trace element distributions. Furthermore, and most importantly, no measured data exists in the literature on atmospheric dust loads over the open ocean -- only estimates extrapolated from adjacent land based sampling sites, calculated concentrations from shipboard measurements or estimates from particle interceptor traps.

The CLIVAR/CO2 Repeat Hydrography program has provided an opportunity to collect and measure surface water column profiles and aerosol concentrations for trace elements.

  suspended particulate Al map
suspended Fe particulate map
  Figure 3 and 4. Contour plots of suspended particulate Al and Fe from the A16N CLIVAR transect sampled in the summer of 2003.
Basin-wide transects of dissolved and particulate trace metal concentrations in solution as well as total and soluble fractions of aerosols can be a powerful tool in constraining ocean-atmosphere carbon models. Not only are many trace elements bioactive (Mn, Fe, Co, Ni, Cu, Zn) but they can also be used as tracers for particle scavenging processes (Al, Mn, Co, Pb) and atmospheric input (Al, Si, Ti and Fe).

Research Goals

The Climate Variability and Predictability program (CLIVAR) is an international, interdisciplinary research effort focused on understanding seasonal to centennial variations in the climate system. The US CLIVAR organization has created a Repeat Hydrography program (Figure 1; http://ushydro.ucsd.edu/repeathydro_map.html ) which, “is driven by the need to monitor the changing patterns of carbon dioxide (CO2) in the ocean and provide the necessary data to support continuing model development that will lead to improved forecasting for oceans and global climate.

Satellite image of airborne dust  
Figure 5. Satellite image of airborne dust generated from a Saharan dust storm.  
The WOCE/JGOFS survey during the 1990s has provided a full depth, baseline data set against which to measure future changes. By integrating the scientific needs in the following five areas, major synergies and cost savings will be achieved. These areas are of importance both for upcoming research programs, such as CLIVAR and the U.S. GCRP Carbon Cycle Science Program (CCSP), and for operational activities such as GOOS and GCOS. In this regard, consensus was reached at the First International Conference on Global Observations for Climate, held in St. Raphael, France in October 1999, that one component of a global observing system for the physical climate/CO2 system should include periodic observations of hydrographic variables, CO2 system parameters and other tracers” (Smith and Koblinsky, 2000; http://ushydro.ucsd.edu/history_justification.html). Within this framework we hope to elucidate the biogeochemical role of dust in the world’s oceans by:
(1) Measuring the temporal and spatial variability and magnitude of aerosol deposition into surface ocean waters as well as its subsequent dissolution.
(2) Continue the work undertaken on previous CLIVAR cruises by mapping the distributions of dissolved and particulate Fe, Al and other trace elements. This will allow for evaluation that aerosol deposition plays in maintaining trace element concentrations.

Data and Preliminary Result
Our role in the CLIVAR/CO2 Repeat Hydrography is to collect, quantify and analyze data for dissolved particulate and aerosol Fe, Al and other trace elements. Research efforts are conducted in conjunction with Dr Chris Measures http://www.soest.hawaii.edu/oceanography/faculty/measures.html ( of the University of Hawaii (shipboard dissolved Al and Fe profiles using flow injection analysis) and Dr William Landing, Florida State University (http://ocean.fsu.edu/faculty/landing/landing.html (aerosol collection and solubility experiments). This has produced large scale data sets that can be used to ground-truth biogeochemical and coupled ocean/atmospheric/climate models. Results have revealed many unknown features regarding the distributions of dissolved and particulate Al and Fe, as well as Fe aerosol chemistry not expressed in existing models.

Trace Element particulate distributions --
Participation in the 2003 CLIVAR U.S. Repeat Hydrography A16 cruise produced a large and comprehensive dataset of trace element distribution in the Northern Atlantic (Figure 2). Samples were collected using a rosette based trace element sampling system. Sixty two vertical profiles sampling 12 depths to 1000 meters were made every 60 Nautical miles. Suspended particulate matter was collected with 0.4 µm, 47mm PCTE TSM filters. Filters were analyzed using energy dispersive X-ray fluorescence methods on a Spectro X-Lab 2000 X-ray Fluorescence Spectrometer. Concentrations of AL, Si, P, S, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Br and Pb have been quantified. Figures 3 and 4 showcase the potential of large, semi-synoptic basin wide transects. Particulate Al and Fe sections from the A16 transect display coherent distributions, with elevated concentrations in surface and intermediate waters beneath the main summertime track of Saharan dust plume. Large summer storms transport Saharan dust to the North Atlantic injecting the surface waters with trace elements (figure 5). Figure 6 displays Aerosol Index for July 26, 2003 from the Goddard Space Flight Center. The red arrow indicates the sample location in relation to a high dust mass event. The inset characterizes Fe aerosol Flux versus Latitude. Further studies are being conducted to constrain the impact of such deposition by investigating aerosol Fe and Al solubility. See Buck et al. G3 Volume 7, number 4, April 2006.