The GEOTRACES Project

  • Dr Roisin Moriarty

Using helium and neon to trace the origins of tracemetals along 40oS

UKGEOTRACES focuses on trace metals and paleoproxies in the south Atlantic along 40o. Helium and neon will be used to identify the source of trace metals (micronutrients), particularly iron and manganese, originating from the Mid Atlantic Ridge. Tritium can be used in conjunction with helium to estimate flux rates away from hydrothermal sources.

UKGEOTRACES_figure1

New data in the Southern Ocean showing a doubling of Fe content in the <0.2micron dissolved fraction close to the MAR on the zero Meridian (Klunder et al, in press) illustrating the significance of the volcanic micronutrient sources of the ocean. (After Klunder et al, 2010 PhD thesis) This figure shows a cross-section of dissolved iron in seawater, with an obvious peak in concentration over the mid-ocean ridge. This data comes from the Southern Ocean, further to the south of us here, and was collected by Maarten Klunder, one of the post-docs onboard, during his PhD work. My hope is to combine such data at 40°S with 3He measurements that confirm where the Fe came from, and will tell us how quickly it is removed from seawater.

As part of the tracer team on the UKGEOTRACES 40o South cruise we did not sample micronutrients (trace metals) during the cruise but chemical elements that allow the identification of the sources of micronutrients. As a noble gas geochemist at the University of Manchester I participated in UK GEOTRACES to collect samples of helium (He), neon (Ne). Noble gases are special as they are not affected by biological or chemical processes only physical processes.

We are specifically interested in 3He with its source at the Mid Atlantic Ridge (MAR). Terrigenic or volcanic 3He is produced on the sea floor in hydrothermal vent areas, which are usually found at sea-floor spreading centers. The MAR is one such area. 3He is known as a tracer because there is not a lot of it – trace amounts – it allows you to trace – follow – it to its original source and to trace – better understand – physical processes such as mixing in the ocean. It is mainly used to understand deep ocean circulation as it is only introduced into the ocean in very deep waters. There are also atmospheric inputs of 3He and in order to separate this signal from terrigenic 3He released from the MAR we also measure 20Ne which comes only from the atmosphere.

Why are we interested in the 3He signal from the MAR? There are very few measurements of micronutrients at depth in the oceans. This means that scientists do not know the distribution and concentrations of most micronutrients in the oceans. Micronutrients are known to be released along mid ocean ridges (MORs). One of the micronutrients that we are very interested in on this cruise is iron (Fe). It was thought at one point that the amount of dissolved Fe supplied to the ocean from MORs was limited as Fe from hydrothermal vents precipitates out of solution very quickly. Work done recently that suggests that there is an increase in dissolved Fe concentration around mid ocean ridges as Fe binding ligands prevent iron from precipitating out of solution. In the oceans ligands usually take the form of organic compounds produced by microorganisms, but little is really known about their composition and origin.

We went to 40o South to document the distribution of micronutrients and to identify the sources of those micronutrients. Noble gases allow us to identify source inputs of micronutrient from the MAR. Sampling was focused around the MAR. Once we have analyzed the noble gas samples from across the entire section this information will help us to identify the direction and extent of the hydrothermal vent plume.

The ANDREX Project

Using tritium and helium to trace water circulation in the Weddell Gyre

ANDREX (ANtarctic Deep water Rates of EXport) focuses on constraining rates of deep-water formation in the Weddell Sea off Antarctica. It is important to understand a number of processes in this region, including ventilation of the deep global ocean and carbon export within the Weddell Sea gyre, to determine the role of the Southern Ocean in global ocean circulation and climate. Tritium will be used to help quantify ventilation age and transit time distribution and helium-3 will be used to help estimate contemporary export of ventilated surface water from the gyre.

Groundwaters

With an increased awareness of environmental issues, there has been a great deal of focus on groundwater systems. These include the determination of groundwater age using tritium and 4He techniques to assess water residence time and its origin. Dating water has application in a variety of areas such as water resource management and pollution tracing, but also provides a critical parameter used to assess rates of change in groundwater chemistry.

Tritium is a radioactive isotope of hydrogen found in the atmosphere that is naturally part of the water molecule. Once in the ground there are no significant tritium sources and as the groundwater becomes older, with a half-life of 12.43yrs, the tritium concentration decreases. By taking a groundwater sample, degassing it and storing it for several months, a portion of any remaining tritium decays in the sample to 3He. This in turn can be precisely measured in the noble gas laboratory, enabling a groundwater tritium date to be calculated. This application is appropriate for groundwaters that are typically less than ~60yrs old, but is also used to identify small additions of young groundwater to older aquifers.
4He in groundwater provides a totally different scale of groundwater ages, from ~1K to 100K years, and is produced by the radioactive decay of naturally occurring U and Th in the sediments and underlying rock strata. The accumulation of 4He in the water is a function of time, and can be used to place relative ages on related groundwaters bodies. Another important use of noble gases in groundwater is the determination of paleotemperature. When groundwater at recharge equilibrates for the last time with the atmosphere, the water temperature determines the concentration of the dissolved noble gases. Knowing the age of the groundwater and its noble gas composition, it is possible to reconstruct how surface temperature has changed in the past.