Award: OCE-1235328

For a number of reasons, the world?s oceans are changing – they are becoming warmer, the pH is dropping as they become more acidic, and in northern waters the salinity is decreasing as glaciers and ice caps melt and river flows increase. Many of these changes are easily seen and measured, but another change is less apparent since it is well below the surface – the amount of oxygen dissolved in the water below the sunlit zone (ca. 200-800 m depth) is decreasing. The reasons for this decrease are a combination of warming waters increasing bacterial respiration as they consume dead organic matter, changes in the way the water circulates and brings oxygen to deeper waters, and in some locations increased phytoplankton (microscopic plants) growth from excess nutrients increasing the flux of organic matter from the surface to the deeper ocean where it is then respired and oxygen decreases. While low oxygen clearly affects fish and other animals in the ocean, it also alters the chemistry of the water. This research project examined how low oxygen affects the cycling of essential (e.g., iron) and toxic (e.g., arsenic) trace elements, but more importantly explored new ways to study and measure the chemistry of low oxygen waters. One of the world?s largest low oxygen zones is off the Peru shelf where high biological productivity due to nutrient inputs from upwelling produces large amounts of organic matter that is then respired in deeper, subsurface waters. This is a great natural laboratory to examine the chemistry of low oxygen waters. In October – December 2013 we used the Research Vessel Thompson to sample a total of 37 stations from top to bottom from Peru to Tahiti (Figure 1; we left from Manta, Ecuador, but sampling was from Peru to Tahiti). Chemists measure various constituents like nutrients and trace elements to examine processes happening in the water column where they sampled, but it is important to realize that physical transport (e.g., ocean currents) can bring chemical constituents from elsewhere and may not be just reflecting processes in the water column of a given station. In other words, while you may measure very low oxygen concentrations at a particular location, the oxygen consumption could have happened elsewhere and the signal transported to a given location. Separating the processes happening at a given location and depth (in situ) from those what happened elsewhere was one of the goals for this project. To do this research we measured dissolved oxygen, and a series of oxidized/reduced compounds: nitrate/nitrite, iodate/iodide, selenite/selenite, and arsenate/arsenite. We also collaborated with other scientists studying iron (reduced iron is Fe(II)), and the radioactive isotopes 228Ra and 234Th. Examples of these data are shown in Figures 2 and 3, with oxygen having a tongue of very low, almost zero, concentrations between 50 and 600 m depth and starting at the coast and going offshore for almost 1000 km (Fig. 2). Iodide (I-, reduced form of iodine; Fig. 3A), nitrite (reduced form of nitrogen; Fig 3B) also have highest concentrations in the low oxygen tongue, but As(III) (reduced form of arsenic; Fig. 3C) does not. This tells us that the oxidation/reduction (redox) conditions in the low oxygen waters are sufficiently strong to allow iodine and nitrogen reduction, but not strong enough to reduce arsenic. This has been observed before and is not unusual, but it is contradicted by the observation in Fig. 3D that there is Fe(II) in these same waters. However, reducing iron to Fe(II) requires stronger reducing/low oxygen conditions; how or where did the Fe(II) form? We hypothesized that the Fe(II) is formed in anoxic sediments in coastal Peru and then transported by currents and diffusion offshore. To check this we used 228Ra data since this isotope has a unique and strong source in sediments, and a plot of Fe(II) vs. 228Ra shows a strong correlation. Further, the radioactive 228Ra allows us to calculate the rate at which this sediment Fe(II) is transported offshore. This shelf flux rate is almost identical to the rate at which Fe(II) would have to be produced or delivered to the offshore waters. Thus, the combination of radioactive tracers and trace element data for reduced or oxidized compounds can be combined to evaluate trace element sources. In a changing ocean it is critical that we use methods like this to evaluate whether changes in the concentrations and chemical forms of biologically essential elements like iron, or toxic ones like arsenic, are due to changes in the water column or in coastal environments, including sediments. Last Modified: 07/14/2017 Submitted by: Gregory A Cutter