User:Jamiedix/sandbox

Ba/Ca ratios as a river outflow proxy

Barium/Calcium ratios
Barium/calcium ratios in planktonic foraminifera are used as a geochemical proxy for constraining past river water discharge into the ocean. This method relies on the assumptions that river water contains much higher Ba/Ca than seawater, and that Ba/Ca incorporation into foraminifera calcium carbonate shells depends only on the Ba/Ca concentration in water. Environmental parameters including temperature, salinity, symbiont photosynthesis, light level, and pH do not effect the Ba concentration.

Uses with δ18Owater
Ba/Ca ratios are often used in combination with the runoff proxy δ18Owater. δ18Owater is dependent on the isotopic fractionation during evaporation, and the isotopic composition of precipitation. Ba/Ca is only dependent on Ba/Ca water concentrations from mixing of riverine fresh water input and seawater, and the efficiency of different foraminifera species to incorporate Ba into their shells. This makes Ba/Ca a particularly useful proxy for salinity and river outflow, and provides a reliable secondary tracer in high precipitation regions such as the Amazon Basin.

Applications
Ba/Ca ratios have been used to reconstruct glacial-interglacial changes in the North Atlantic. Nutrient concentrations in the thermocline intermediate water of the North Atlantic were lower than present day values from the presence of nutrient depleted Glacial North Atlantic Intermediate water. This water was replaced by nutrient rich waters from mid-depth Atlantic water. The variation in Ba/Caforam values indicate an increase in meltwater discharge between 12.4-11.3 kya.

Ba/Ca and δ18Owater were used as an indicator of deglacial meltwater discharge into the western Arctic Ocean. Elevated Ba/Ca ratios around 11.8 kya are coincident with enhanced weathering and erosion from deglaciation. This is believed to correlate with the retreat of the Laurentide Ice Sheet, and factors contributing to the onset of the Younger Dryas.

Other areas of Ba/Ca research include finding a link between Indian Ocean sea surface temperature and precipitation of southeastern Africa, West African monsoon variability during the last glacial period , Indian Summer Monsoon variation during glacial and interglacial periods , and many others.

Methods
Ocean cores are sieved to remove clays and separate foraminifers. Common species for Ba/Ca analysis are Orbulina universa, Globigerina bulloides, Globigerinoides ruber, Globigerinoides sacculifer, neogloboquadrina pachyderma, and neogloboquadrina dutertrei. After separation, the tests undergo an extensive oxidative and reductive cleaning process to remove barite, organic matter, and clay. Sample analysis is typically performed on a multi-element inductively coupled plasma mass spectrometer (ICP-MS). Ba/Caseawater ratios are then calculated using two basic relationships. Barium partitioning into foraminifera shells is described by a partition coefficient, DBa.

DBa = Ba/Caforam / Ba/Caseawater

Each species has a specific associated partition coefficient.

The experimentally determined combined partition coefficient for planktic foraminifers is 0.19 ± 0.05  and 0.22 ± 0.02.

Seawater Ba/Ca can be calculated using a linear regression.

Ba/Cashell = 0.149(±0.05)(Ba/Caseawater)

Limitations
River outflow and salinity information resulting from the trace element composition of planktonic foraminifera is limited by the organisms ability to accurately record seawater chemistry in their calcite shell matrix. There is also variation among different foraminifera species due to the depth at which calcification occurs, and their ability to withstand low salinity and high turbulence conditions.

Conflicting evidence
Laser ablation has shown that it may be possible for Ba/Ca to vary from the inside to the outside of the calcite shell. However, this variation is likely from barite contamination from insufficient cleaning of tests. Alkaline diethylenetraimine-pentaacetic acid (DTPA) can be used to remove barite before analysis, and sequential dissolution experiments with DTPA cleaned tests has shown a homogeneous Ba/Ca distribution.