the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Large salinity gradient and diagenetic changes in the northern Indian Ocean dominate the stable oxygen isotopic variation in Globigerinoides ruber
- 1Micropaleontology Laboratory, National Institute of Oceanography, Goa, India
- 2Banaras Hindu University, Varanasi, Uttar Pradesh, India
- 3Indian Institute of Technology, Roorkee, India
- 4National Center for Polar and Ocean Research, Goa, India
- 5MARUM, University of Bremen, Bremen, Germany
- 1Micropaleontology Laboratory, National Institute of Oceanography, Goa, India
- 2Banaras Hindu University, Varanasi, Uttar Pradesh, India
- 3Indian Institute of Technology, Roorkee, India
- 4National Center for Polar and Ocean Research, Goa, India
- 5MARUM, University of Bremen, Bremen, Germany
Abstract. The application of stable oxygen isotopic ratio of surface dwelling Globigerinoides ruber (white variety) (δ18Oruber) to reconstruct past hydrological changes requires precise understanding of the effect of ambient parameters on δ18Oruber. The northern Indian Ocean, with huge freshwater influx and being a part of the Indo-Pacific Warm Pool, provides a unique setting to understand the effect of both the salinity and temperature on δ18Oruber. Here, we use a total of 400 surface samples (252 from this work and 148 from previous studies), covering the entire salinity end member region, to assess the effect of seawater salinity and temperature on δ18Oruber in the northern Indian Ocean. The analyzed surface δ18Oruber very well mimics the expected δ18O calcite estimated from the modern seawater parameters (temperature, salinity and seawater δ18O). We report a large diagenetic overprinting of δ18Oruber in the surface sediments with an increase of 0.18 ‰ per kilometer increase in water depth. The salinity exerts the major control on δ18Oruber (R2 = 0.63) in the northern Indian Ocean, with an increase of 0.29 ‰ per unit increase in salinity. The relationship between temperature and salinity corrected δ18Oruber (δ18Oruber - δ18Osw) in the northern Indian Ocean [T= -0.59*(δ18Oruber - δ18Osw) + 26.40] is different than reported previously based on the global compilation of plankton tow δ18Oruber data. The revised equations will help in better paleoclimatic reconstruction from the northern Indian Ocean.
Rajeev Saraswat et al.
Status: open (until 31 Aug 2022)
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RC1: 'Comment on essd-2022-107', Anonymous Referee #1, 15 Jul 2022
reply
Saraswat et al. present an extensive core-top compilation of new and published δ18O values of the species Globigerinoides ruber. G. ruber is the most commonly measured planktic species and the new data set has the potential to improve the coverage in the Indian Ocean and can complement existing global compilations (i.e., Waelbroeck et al. 2005). The data set itself would benefit from additional documentation (data source, sampling depth, δ13C of G. ruber) that can be added with minimal effort. As outlined below, the scientific analysis and interpretation of the data need revision:
Title/L18, 20, 23, 25, 34…This is probably only a wording issue but “salinity” is not influencing or controlling the δ18O of carbonate, δ18O of sea water is. The authors probably use “salinity” because salinity is often reconstructed from δ18O of seawater and because both parameters are locally highly correlated and influenced by precipitation and evaporation. However, I think the authors should be precise. Salinity and δ18O are related but can be decoupled to some extent, i.e., if the source of the freshwater endmember changes, the same salinity might be associated with different δ18O seawater values. This is demonstrated by the large scatter of the the δ18O-salinity relationships in Figure 8 and the different freshwater endmembers in the Arabian Sea (-2.84 ‰) and the BOB (-4.23 ‰).
L39: There are earlier examples of δ18O seawater reconstructions based on G. ruber that should be cited here including Wang et al. (1995), Kallel et al. (1997) and Schmidt et al. (2004).
L44: I disagee with this statement. The relationships between the physicochemical conditions and foraminiferal δ18O seems to be quite constant since we have a very good agreement between laboratory derived equations and field data when the ambient conditions of shell formation are known. The reasons why there are offsets in different basins is that, particularly when core tops are used, the exact conditions of shell formation (δ18O sea water, light intensity, temperature, depth of calcification, seasonal flux, pH…) are not well constrained.
L113/Methods: I suggest to show a table with all the relevant information, the expeditions, number of samples, region etc.
L131: There seems to be a small discrepancy between the values obtained from published sources and the original data. According to PANGAEA and the appendix in the cited thesis by Sirocko (1989), the core top value for MD76-135 is -1.81 ‰ (this paper: -1.89 ‰) and for MD76-136 -1.78 ‰ (this paper: -1.82 ‰). Perhaps this offset is just due to a different sampling depth or averaging. I have not checked the other sites. Please check and document the sampling depth in the core and the actual source of the data (table in paper, supplement or data base with doi/link)
MD76-136: https://doi.org/10.1594/PANGAEA.77485
MD76-135: https://doi.org/10.1594/PANGAEA.77484
Sirocko (1989): https://doi.org/10.2312/reports-gpi.1989.27
Figure 1, 2, 7: The maps have been produced with OceanDataView. Please give proper credit to the author of the software and cite Schlitzer, R., Ocean Data View, https://odv.awi.de, 2018. What kind of interpolation technique has been used to produce the maps?
L146: State the laboratory where the samples have been measured.
Figure 4: The relation of δ18O with water depth is interesting. To what extend is this relation influenced by the fact that shallow sites are more exposed to freshwater input (=lower δ18O) than the more remote deeper sites? To isolate the effect of water depth, it might help to plot the difference to the predicted δ18O vs water depth rather than absolute δ18O.
Figure 5 & 6: What is the purpose of showing separate relations with temperature and salinity (must be δ18O of sea water, see above) when we know that both factors influence the δ18O of foraminiferal carbonate? Temperature and salinity are also spatially related and it is impossible to quantify the relative influence of both factors in the plots. In my opinion, Figures 5 and 6 and the corresponding discussion can be omitted.
Figure 9: I suggest to have the same ranges/limits for x and y axis and to indicate the 1:1 relation with an additional line. Is the slope of the shown regression statistically significantly different from 1? To assess whether the derived linear equations will help to improve paleoclimatic reconstructions (as stated in L27), it would be important to quantify the error of the regression equations.
L155: Salinity from the World Ocean Atlas is provided on a grid. How exactly was salinity estimated at the core location? From the nearest grid point?
L218/Discussion: It should be mentioned that stratigraphic information is not provided for most of the core tops. Core top sediments can represent older time slices when the sedimentation rates are low or when older sediments are exposed due to erosional processes. This does not matter so much if the Holocene is present and stable. However, in the Indian Ocean large Holocene δ18O variations are expected due to variations in Monsoon precipitation.
L 245: Explain how mixed layer depth has been defined/determined?
L246: Provide more context. Why has the O. universa low light equation been used if G. ruber δ18O is better described with the high-light equation (see Thunell et al. 1999)? How big are the constant offsets to the published equations?
L247/Discussion/Figure 9: It would be very interesting to see a map of the residual δ18O (predicted-observed). Any spatial pattern in the residuals might point to the environmental/ecological reason for the deviations from expected δ18O. For example, the chlorophyll maximum is very shallow in the coastal areas of the BOB and the Arabian Sea whereas it is deep in the central Indian ocean. If G. ruber exploits the chlorophyl maximum as a food source (Fairbanks and Wiebe, 1980) it might record higher δ18O due to lower temperatures and lower light levels (i.e., Spero et al., 1997). Likewise, the deeper central areas should show lower sedimentation rates and stronger signals from dissolution and sediment mixing.
L270. The authors attribute the increase of δ18O with water depth to diagenesis and partial dissolution. This is certainly a possible explanation. However, it seems very likely that the deep basins receive much less terrigenous matter than the shallow slope/shelf sites and that the sedimentation rates are significantly lower in the deep parts of the Indian Ocean. When the sedimentation rates are lower than ~2 cm/kyr (quite common in deep pelagic basins) it is likely that the the Holocene is mixed with isotopically heavier material from the last glacial/deglaciation (Broecker 1986), which might also explain the relationship of δ18O with water depth. Is anything known about the sedimentation rates in the deep basins?
L291-293: Unfortunately, δ13C of G. ruber has not been not reported and shown along with δ18O. As DIC in river water is very depleted in carbon-13, since it derives from the remineralized organic matter, it provides additional information on freshwater input (see for example Pastouret et al. 1978). If freshwater from river input is the reason for some of the low δ18O values in G. ruber in the BOB, δ13C values should be low as well. Since δ18O and δ13C of foraminiferal carbonate are always measured together it should be straightforward to add δ13C of G. ruber to the data set.
L306-313: Please review and revise the reasons for the different slopes in the North Atlantic and the Indian Ocean. The higher slope of the δ18O/salinity relationship in the North Atlantic is due to the fact that precipitation/freshwater in high latitudes is depleted in oxygen-18 (~-35 ‰) compared to tropical areas (~-5 ‰) (see for example Figure 10 in Rozanski et al., 1993).
L324: Except for upwelling regions, the variations in surface water pH are indeed small. However, the pH at the foraminiferal shell is strongly influenced by light intensity in the presence of symbionts (see Jorgensen et al. 1985, Fig 6). Small variations in turbidity or calcification depth can therefore influence the stable isotope ratio of G. ruber via the pH effect.
References
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Fairbanks, R. G. and Wiebe, P. H.: Foraminifera and chlorophyll maximum: vertical distribution, seasonal succession, and paleoceanographic significance, Science (New York, N.Y.), 209, 1524–1526, https://doi.org/10.1126/science.209.4464.1524, 1980.
Jørgensen, B. B., Erez, J., Revsbech, P., and Cohen, Y.: Symbiotic photosynthesis in a planktonic foraminiferan, Globigerinoides sacculifer (Brady), studied with microelectrodes1, Limnol. Oceanogr., 30, 1253–1267, https://doi.org/10.4319/lo.1985.30.6.1253, 1985.
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Sirocko, F.: Zur Akkumulation von Staubsedimenten im nördlichen Indischen Ozean; Anzeiger der Klimageschichte Arabiens und Indiens, Berichte - Reports, Geologisch-Paläontologisches Institut und Museum (Kiel, Univ.). Berichte. 27, 27, Geol.-Paläont. Inst. u. Museum Univ, Kiel, 185 pp., 1989.
Spero, H. J., Bijma, J., Lea, D. W., and Bemis, B. E.: Effect of seawater carbonate concentration on foraminiferal carbon and oxygen isotopes, Nature, 390, 497–500, https://doi.org/10.1038/37333, 1997.
Thunell, R., Tappa, E., Pride, C., and Kincaid, E.: Sea-surface temperature anomalies associated with the 1997–1998 El Niño recorded in the oxygen isotope composition of planktonic foraminifera, Geology, 27, 843, https://doi.org/10.1130/0091-7613(1999)027<0843:SSTAAW>2.3.CO;2, 1999.
Waelbroeck, C., Mulitza, S., Spero, H., Dokken, T., Kiefer, T., and Cortijo, E.: A global compilation of late Holocene planktonic foraminiferal δ18O: relationship between surface water temperature and δ18O, Quaternary Science Reviews, 24, 853–868, https://doi.org/10.1016/j.quascirev.2003.10.014, 2005.
Wang, L., Sarnthein, M., Duplessy, J.-C., Erlenkeuser, H., Jung, S., and Pflaumann, U.: Paleo sea surface salinities in the low-latitude Atlantic: The δ 18 O record of Globigerinoides ruber (white), Paleoceanography, 10, 749–761, https://doi.org/10.1029/95PA00577, 1995.
Rajeev Saraswat et al.
Data sets
Oxygen isotopic ratio of Globigerinoides ruber (white variety) in the surface sediments of the northern Indian Ocean Saraswat, Rajeev; Suokhrie, T; Naik, Dinesh Kumar; Singh, Dharmendra Pratap; Saalim, S M; Salman, M; Kumar, G; Bhadra, S R; Mohtadi, Mahyar; Kurtarkar, S R; Maurya, A S (2022) https://www.pangaea.de/tok/59190adf9e4facf7ebb9ad555c0bce58a9a72bd9
Rajeev Saraswat et al.
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