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Ba/Ca ratios as a river outflow proxy
Barium/Calcium ratios
[edit]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.[1] Environmental parameters including temperature, salinity, symbiont photosynthesis, light level, and pH do not effect the Ba concentration.[2]
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[3][4]. Ba/Ca is only dependent on Ba/Ca water concentrations from mixing of riverine fresh water input and seawater[1], 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
[edit]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.[5]
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.[6]
Other areas of Ba/Ca research include finding a link between Indian Ocean sea surface temperature and precipitation of southeastern Africa[7], West African monsoon variability during the last glacial period[8], Indian Summer Monsoon variation during glacial and interglacial periods[9], and many others.
Methods
[edit]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.[10] 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.[2]
DBa = Ba/Caforam / Ba/Caseawater
Each species has a specific associated partition coefficient.[2]
| Species | DBa |
|---|---|
| G. bulloides | 0.17±0.04 |
| O. universa | 0.17 ± 0.02 |
The experimentally determined combined partition coefficient for planktic foraminifers is 0.19 ± 0.05[11] and 0.22 ± 0.02[12].
Seawater Ba/Ca can be calculated using a linear regression.[12][11][2]
Ba/Cashell = 0.149(±0.05)(Ba/Caseawater)
Limitations
[edit]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.[2] 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.[1]
Conflicting evidence
[edit]Laser ablation has shown that it may be possible for Ba/Ca to vary from the inside to the outside of the calcite shell.[13] However, this variation is likely from barite contamination from insufficient cleaning of tests. [2]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.[11]
References
[edit]- ^ a b c Bahr, André (2012). "Comparison of Ba/Ca and δ18OWATER as freshwater proxies: A multi-species core-top study on planktonic foraminifera from the vicinity of the Orinoco River mouth". Earth and Planetary Science Letters. 383: 45–57.
- ^ a b c d e f Hönisch, Bärbel (2011). "Planktic foraminifers as recorders of seawater Ba/Ca". Marine Micropaleontology. 79: 52–57 – via Elsevier Science Direct.
- ^ Conroy, Jessica (2014). "Constraints on the salinity–oxygen isotope relationship in the central tropical Pacific Ocean" (PDF). Marine Chemistry.
- ^ Benson, Paillet (2002). "HIBAL: a hydrologic-isotopic-balance model for application to paleolake systems". Quaternary Science Review. 21: 1521–1539 – via Elsevier.
- ^ Hall, Jenney M (2002-07-11). "Barium and Lithium in Foraminifera: Glacial-Interglacial Changes in the North Atlantic". etd.lsu.edu. Retrieved 2017-03-01.
- ^ Hall, Jenney M.; Chan, L.-H. (2004-03-01). "Ba/Ca in Neogloboquadrina pachyderma as an indicator of deglacial meltwater discharge into the western Arctic Ocean". Paleoceanography. 19 (1): PA1017. doi:10.1029/2003PA000910. ISSN 1944-9186.
- ^ Weldeab, Syee; Lea, David W.; Oberhänsli, Hedi; Schneider, Ralph R. (2014-09-15). "Links between southwestern tropical Indian Ocean SST and precipitation over southeastern Africa over the last 17 kyr". Palaeogeography, Palaeoclimatology, Palaeoecology. 410: 200–212. doi:10.1016/j.palaeo.2014.06.001.
- ^ Weldeab, Syee (2012-04-27). "Bipolar modulation of millennial-scale West African monsoon variability during the last glacial (75,000–25,000 years ago)". Quaternary Science Reviews. 40: 21–29. doi:10.1016/j.quascirev.2012.02.014.
- ^ Evans, David; Bhatia, Rehemat; Stoll, Heather; Müller, Wolfgang (2015-08-01). "LA-ICPMS Ba/Ca analyses of planktic foraminifera from the Bay of Bengal: Implications for late Pleistocene orbital control on [[monsoon]] freshwater flux". Geochemistry, Geophysics, Geosystems. 16 (8): 2598–2618. doi:10.1002/2015GC005822. ISSN 1525-2027.
{{cite journal}}: URL–wikilink conflict (help) - ^ Mashiotta, T.A. (1997). "Experimental determination of Cd uptake in shells of the planktonic foraminifera Orbulina universa and Globigerina bulloides: implications for surface water paleoreconstructions". Geochimica et Cosmochimica Acta. 61: 4053–4065.
- ^ a b c Lea, D.W. (1991). "Barium in planktonic foraminifera". Cosmochimica Acta. 60: 3143–3149.
- ^ a b Hall, J.M. (2004). "Ba/Ca in Neogloboquadrina pachyderma as an indicator of deglacial meltwater discharge into the western Arctic Ocean". Paleoceanography2. 19.
- ^ Hathorne, James (2009). "Environmental versus biomineralization controls on the intratest variation in the trace element composition of the planktonic foraminifera G. inflata and G. scitula". Paleoceanography. 24.