Biogeochemical impacts of a western iron source in the Pacific Equatorial Undercurrent
Profiles of total acid soluble and particulate iron in the western equatorial Pacific (~155°E) have a maximum associated with the equatorial undercurrent (EUC) (Slemons et al., 2010, 2012). This maximum is mostly in the form of particulate iron that appears to originate from rivers and sediments along the northeastern continental margin of Papua New Guinea. There does not appear to be a corresponding maximum for filterable Fe.
We conducted a model simulation (OPA/PISCES) in which this western iron source imposed in the EUC was transported to the east and we evaluated its impact on biogeochemical distributions. We treated all of the total acid soluble iron as if it was 100% bioavailable. A control simulation without the enhanced iron source was run for reference. In the source runs the concentrations of iron decrease from west to east, primarily due to scavenging. The western iron source can explain the maxima in total iron (dissolved plus particulate) observed at 140°W. But the control runs did a better job of reproducing the climatological fields of NO3 and chlorophyll. With the source runs NO3 was much lower and chlorophyll is much higher than expected. Diatom production was also excessively enhanced. There were a few examples where the source runs reproduced the data better such as zonal gradients of surface nitrate along the equator and the meridional gradients of primary productivity and carbon export production.
Overall, the implications are that most of the total acid soluble iron in the EUC is not bioavailable to phytoplankton in the eastern equatorial Pacific. Even though there is a maximum in acid soluble iron associated with the EUC not all of this iron is available for biological uptake.
For more detail see:
Slemons et al (2009) Deep-Sea Research I
Slemons et al (2010) Global Biogeochemical Cycles.
Slemons et al (2012) Marine Chemistry
Natural and Anthropogenic Ocean Acidification
There are three sources for CO2 acidification of the ocean
1. Classic “ocean acidification” – ocean uptake of atmospheric CO2
2. Naturally eutrophic ocean acidification – mostly aerobic respiration – low O2 sources
3. Anthropogenic eutrophic – driven by nutrients of pollution origin – e.g. Hood canal
These can be difficult to distinguish but it is important to do so as they vary on different temporal and spatial scales.
One result of climate change is that we have totally reversed the flux of CO2 between the atmosphere and ocean. In preindustrial times the flux was from the ocean to the atmosphere. Now it is the reverse. About 30% of the CO2 produced by mankind is going into the ocean. At time series locations like Hawaii and Bermuda we see clearly that the ocean CO2 is increasing and the pH is decreasing. The saturation with respect to carbonate minerals is decreasing.
To complicate the issue there are locations like Puget Sound and San Francisco Bay where upwelling of high CO2 water (and low O2 and high NO3) from offshore brings water with natural ocean acidification.
We first observed this in a time series of data for temperature, salinity, nitrate, and carbonate chemistry from September 2011 to July 2013 at the University of Washington’s Friday Harbor Laboratories. Seawater conditions at Friday Harbor are high nitrate-low chlorophyll (HNLC), with average nitrate and pCO2 concentrations of 25 mol L-1 and 700 atm L-1 (pH 7.80). The high nitrate and pCO2 originate from the high values for these parameters in the source waters to the Salish Sea from the California Undercurrent (CU). These properties are due to natural aerobic respiration in the region where the CU originates, which is the oxygen minimum zone in the eastern tropical North Pacific. Similar data were observed in San Francisco Bay at a time series at the Exploratorium science museum.
We calculated that the anthropogenic “ocean acidification” contribution to DIC in the source waters of the CU was 36 mol L-1. This contribution ranged from 13% to 22% of the total increase in DIC, depending on which stoichiometry was used for C/O2 ratio (Redfield vs. Hedges). The remaining increase in DIC was due to natural aerobic respiration.
James Murray has been a faculty member in the School of Oceanography at the University of Washington since 1973. He became Emeritus Professor in September 2013. He is a Senior Fellow in the Joint Institute for Study of the Atmosphere and Oceans (JISAO) and an Adjunct Professor, Department of Chemistry, University of Washington. He was the Founding Director of the Program on Climate Change in 2001.
His major scientific interests are the distributions and origin of iron and other trace elements in the equatorial undercurrent of the Pacific, nitrogen cycling in the Black Sea and impacts of ocean acidification on marine biota. Previous interests have included the surface chemistry of solid phases and the mechanism of scavenging in the ocean, diagenesis of organic matter and gases in marine sediments, particle reactive radioisotopes as tracers for particle cycling and new and export production. He also works on energy issues from a perspective that they represent an uncertainty on climate change.
His educational training was at the University of California (1968, B.A. Geology); Massachusetts Institute of Technology/Woods Hole Oceanographic Institution Joint Program (1973, Ph.D. Chemical Oceanography).
He was President of the Ocean Science Section of the American Geophysical Union from 2013 to 2015. He was the lead organizer of the 1988 R/V Knorr Black Sea Expedition and the 1992 JG OFS Process Study in the Equatorial Pacific (EqPac). He led formation of an Ocean Acidification Research Lab at the UW Friday Harbor Laboratories from 2010 to 2013.
He is a Fellow of AAAS and AGU and was a Fulbright Scholar in 2002. He was a Resident Fellow at the Rockefeller Foundation Bellagio Estate in June 2016.