Hydrothermal contributions to micronutrient budgets

In the past few decades we have come to the realization that the availability of certain trace metals, known as “micronutrients” play a constraining role in marine biological productivity in large regions of the ocean. Until very recently, the oceanic distribution of one of the most important of these, iron, was thought to be controlled by the atmospheric deposition of dust at the ocean surface. This paradigm was overturned in the past decade by the recognition that the injection of metals in deep submarine hot springs plays a dominant role in maintaining the global ocean inventory of iron and other elements. One motivation of this program was to document and quantify this influence.

Our strategy was to measure the distribution of a rare isotope of helium, namely ³He, in seawater along a zonal section in the tropical South Pacific and relate its abundance to the trace metals of interest. The distribution of ³He has been mapped throughout the world’s ocean, and beautifully highlights the injection of this isotope from hydrothermal hot springs on the ocean floor and many sites around the globe. The attraction of ³He is

1. It is very abundant in the earth’s mantle. Thus, ³He is a strong indicator of interaction with it.
2. ³He is an extremely rare isotope of a very rare gas, so any additions are “visible” over great distances.
3. ³He is a stable isotope and does not decay with time.
4. Helium is a noble gas, so it is not altered by chemical or biological processes. Thus it is unambiguously preserved.
5. We have used observations of the distribution of this isotope, along with ocean circulation models to determine the integrated global hydrothermal flux of this isotope to be 500 moles/year.

Figure 1 is a section of this isotope along the cruise track (depth vs longitude, west ion the left, ocean surface on top). There is a mid-depth plume emanating from the East Pacific Rise (a mid-ocean ridge where seafloor spreading occurs) at the center of the section (also shown as dashed line on the inset map). It extends many thousands of kilometers westward. In fact, the distribution of dissolved iron (as measured by our colleagues on this cruise) shows a remarkably similar picture. Figure 2, taken from a published Nature paper, shows how tightly they correlate: except for Station 18, which lies directly above the injection site, the data fall on a straight line. This demonstrates that the hydrothermal dissolved iron is affected only by dilution away from the East Pacific Rise. In this way, we have determined the ratio of iron to ³He in the hydrothermal effluent (the slope of the relationship in Figure 2), which allows us to compute the Global Hydrothermal Iron Flux of approximately 4 x 10⁹ Moles per year. This number is important for establishingmoer its impact on global marine productivity.

The impact of the Peruvian Upwelling System

The Peruvian Upwelling System, a region of enhanced biological productivity fueled by deep-water nutrients brought up by wind-induced coastal upwelling, supports a regionally important fisheries industry. Although a more-or-less persistent feature, inter-annual variations in its intensity most notably  known as El Niño/La Niña events result in marked declines and booms in fisheries yields. Understanding in a quantitative way how the subsurface nutrient-rich source waters couple with the upwelling and how this consequently drives off-shore marine productivity is an important objective. The primary challenge is that variations in the driving forces for this upwelling system occur on a wide range of time-scales, spanning months to decades and longer. Tracers offer a unique integrative measure of these processes, and this is part of our approach to the problem. Our goal was to document the zonal distributions of three complementary tracer groups to form a basis for future models. These tracers probe a range of time-scales:

1. Tritium-helium dating uses the radioactive decay of an extremely rare isotope of hydrogen (tritium) to an equally rare isotope of helium (³He) to establish the time water has been isolated from the atmosphere, and works on time-scales ranging from a few weeks to a few decades.
2. Dissolved noble gases (helium, neon, argon, krypton, and xenon) respond to upwelling on time-scales ranging from a few weeks (for helium) to several months (for xenon).
3. Radiocarbon is affected by a very slow exchange (approaching a decade) with the atmosphere.

Figure 3 is a plot of surface water values for these tracers as a function of longitude. Note the strong and significant increase in tracer anomalies as one approaches the Peruvian shelf on the right, strongly correlating with upwelling nutrients (NO₃ and SiO₄ shown as examples). The subsurface distributions (not shown) demonstrate a clear delineation of the water and material pathways that feed the system.

Last Modified: 03/30/2018
Modified by: William J Jenkins