Oxygen isotope–salinity relationships of discrete oceanic regions from India to Antarctica vis-à-vis surface hydrological processes (original) (raw)
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Oxygen isotope/salinity relationship in the northern Indian Ocean
Journal of Geophysical Research, 2001
We analyze the surface •5•80 -salinity relationships of the Bay of Bengal and the Arabian Sea, in the northern Indian Ocean, known for their contrasting hydrological conditions. New measurements of these tracers show a very low •5•80 -salinity slope associated with the strong dilution in the Bay of Bengal, but a slope more typical of this latitude in the Arabian Sea. Although this region is marked by a complex monsoonal regime, numerical modeling using a box model and a general circulation model is able to capture the •5•SO -salinity slope and its geographical variation. Both models clearly show that the low •5•SO -salinity slope is due to the evaporation-minus-precipitation balance, with an important contribution of the continental runoff in the Bay of Bengal. Although the low value of these slopes (-0.25) makes past salinity reconstructions uncertain, insight into the Last Glacial Maximum conditions shows a probable stability of these slopes and limited error on paleosalinity. 1. Introduction Hydrological studies of the northern Indian Ocean have focused on the flux divergence of vapor in order to refine both the mechanism and the vapor source of the monsoon precipitation over India. Cadet and Reverdin [1981] estimated the Arabian Sea contribution to the monsoonal precipitation to be about 20%, the remainder originating from the Southern Hemisphere. They showed a large increase in the evaporation-minus-precipitation Among the hydrological characteristics of the oceanic waters the •SO content (the H2•80/H2•60 ratio R, hereafter expressed as the deviation/5 with respect to the Vienna standard mean ocean water (VSMOW) value, in permil: 15•SO = (R/RsMow-1)1000) is rarely used because of the measurement difficulties. Together with the salinity and other tracers, it can serve to characterize a water mass [e.g., Craig and Gordon, 1965; Pierre et al., 1994] (E-P) balance over the Arabian Sea at the onset of the monsoon, and to disentangle the different contributions to a water mixture with no counterpart in the Bay of Bengal. This E-P budget is very [e.g., Ostlund and Hut, 1984]. The •5
Spatiotemporal variations of the δ18O–salinity relation in the northern Indian Ocean
Deep Sea Research Part I: Oceanographic Research Papers, 2010
A new data set of oxygen isotopic composition (d 18 O) and salinity (S) of surface and sub-surface waters of the northern Indian Ocean, collected during the period 1987-2009, is presented. While the results are consistent with positive P À E (excess of precipitation over evaporation) over the Bay of Bengal and negative P À E over the eastern Arabian Sea, a significant spatiotemporal variability in the slope (also intercept) of the d 18 O-S relation is observed in the Bay; the temporal variability is difficult to discern in the Arabian Sea. The slope and intercept are positively and negatively correlated, respectively, with the annual rainfall over India, a rough measure of river runoff. Both the slope and intercept appear to be sensitive to rainfall; the slope (intercept) is higher (lower) during years of stronger monsoon. The observed variability in the d 18 O-S relation implies that caution needs to be exercised in paleosalinity estimations, especially from the Bay of Bengal, based on d 18 O of marine organisms.
Acta Geologica Sinica - English Edition, 2012
Paired stable oxygen isotope and Mg/Ca analyses in calcite tests of mixed-layer dwelling planktic foraminifer Globigerinoides ruber, has been used to reconstruct equatorial Indian Ocean δ 18 O of seawater (δ 18 O sw) over the last ~137 kyr. On the basis of ice volume corrected δ 18 O sw (δ 18 O sw-ivc), relative changes in sea surface salinity (SSS) have been estimated. The SSS estimates suggest three episodes of higher SSS (131-113 kyr BP, 62-58 kyr BP, and 30-24 kyr BP) within the last glacial period as compared to the present. The SSS comparison between the interglacial episodes reveals that the surface seawater over the core site was significantly saltier during penulitmate interglacial than the Holocene. We suggest that the evolution of seasonal insolation gradient between the Indian monsoon areas and the equator over the investigated time interval was instrumental in shaping the strength of Indian winter and summer monsoons that left their imprints on the equatorial Indian Ocean SSS via fresh water input and wind-induced mixing. The study shows that the insolation difference between northern latitudes and equator during winter time affects the monsoon strength in the Indian region, especially during cold intervals.
Constraints on the salinity–oxygen isotope relationship in the central tropical Pacific Ocean
Please cite this article as: Conroy, Jessica L., Cobb, Kim M., Lynch-Stieglitz, Jean, Polissar, Pratigya J., Constraints on the salinity-oxygen isotope relationship in the central tropical Pacific Ocean, Marine Chemistry (2014), Abstract Uncertainties surround the relationship between salinity and the stable isotopic composition of seawater, largely due to a dearth of modern seawater isotope data. Here we report 191 new, paired measurements of salinity and seawater oxygen isotopes ( 18 O sw ) taken from the central tropical Pacific in May 2012, from the surface to 4600 m depth. We observe significant correlations between 18 O sw and salinity across the study region, with slopes ranging from 0.23-0.31‰/psu for the mixed layer, and 0.35-0.42‰/psu for waters between the mixed layer and 500 m depth. When considering 18 O sw -salinity across averages of individual water masses in the region, slopes range from 0.21-0.40 ‰/psu, albeit with appreciable scatter. Surface salinity and 18 O sw data corresponding to the North Equatorial Countercurrent are significantly higher than previously observed, which we attribute to a weak westerly current and dry conditions in the region during the May 2012 cruise. Subsurface (80-500m) salinity values from 2012 are significantly lower than corresponding values from pre-existing regional data, highlighting a different latitudinal sampling distribution, while subsurface 18 O sw is not significantly different. Thus, in May 2012, 18 O sw in this region could not be used to distinguish between subsurface water masses of different salinities. Unlike other regions where the surface 'freshwater endmember' is close to the 18 O value of regional precipitation, the freshwater endmember implied by our dataset (-10.38‰) is consistent with a strong evaporative influence. Paired 18 O-D values of precipitation and surface seawaters have similar slopes (5.0, 5.1), and relatively low intercepts (1.4, 0.8) indicating isotopic variability in both reservoirs is also partly controlled by evaporation.
Simulation of Oxygen Isotopes in a Global Ocean Model
Use of Proxies in Paleoceanography, 1999
We incorporate the oxygen isotope composition of seawater δ 18 O w into a global ocean model that is based on the Modular Ocean Model (MOM, version 2) of the Geophysical Fluid Dynamics Laboratory (GFDL). In a first experiment, this model is run to equilibrium to simulate the present-day ocean; in a second experiment, the oxygen isotope composition of Antarctic Surface Water (AASW) is set to a constant value to indirectly account for the effect of sea-ice. We check the depth distribution of δ 18 O w against observations. Furthermore, we computed the equilibrium fractionation of the oxygen isotope composition of calcite δ 18 O c from a paleotemperature equation and compared it with benthic foraminiferal δ 18 O. The simulated δ 18 O w distribution compares fairly well with the GEOSECS data. We show that the δ 18 O w values can be used to characterize different water masses. However, a warm bias of the global ocean model yields δ 18 O c values that are too light by about 0.5 ‰ above 2 km depth and exhibit a false vertical gradient below 2 km depth. Our ultimate goal is to interpret the wealth of foraminiferal δ 18 O data in terms of water mass changes in the paleocean, e.g. at the Last Glacial Maximum (LGM). This requires the warm bias of the global ocean model to be corrected. Furthermore the model must probably be coupled to simple atmosphere and sea-ice models such that neither sea-surface salinity (SSS) nor surface δ 18 O w need to be prescribed and the use of present-day δ 18 O w-salinity relationships can be avoided.
Earth and Planetary Science Letters, 2004
We provide two new determinations of the oxygen isotopic composition of seawater during the last glacial maximum (LGM). High-resolution oxygen isotopic measurements were made on interstitial waters from Ocean Drilling Program (ODP) Sites 1168 and 1170 in the southeast Indian Ocean sector of the Southern Ocean. We use a diffusion–advection numerical model to calculate the glacial–interglacial change in bottom-water δ18Osw
Paleoceanography, 1995
Numerous studies have shown that delta18O records from benthic and planktonic foraminifera, primarily a proxy of global ice volume variations, reflect Milankovitch periodicities. To study climatic response to orbital forcing at Ocean Drilling Program site 758, we have generated continuous delta18O and delta13C records from a single benthic foraminiferal species Cibicides wuellerstorfi for the last 3.6 m.y. and extended the planktonic foraminiferal isotope records of Farrell and Janecek (1991) (0-2.5 Ma, based on Globigerinoides sacculifer) to 3.6 Ma (Chen, 1994). We then constructed an age model by matching, correlating and tuning the benthic delta18O record to a model simulation of ice volume (Imbrie and Imbrie, 1980). The filtered 41- and 23-kyr signals based on the resultant astronomically tuned age model are highly correlated to obliquity (r=0.83) and precession (r=0.75), respectively. Although derived with methodology different from Shackleton et al. (1990) and Hilgen (1991a,b)...
Paleoceanography, 2011
1] An isotope-enabled ocean-atmosphere general circulation model (GISS ModelE-R) is used to estimate the spatial gradients of the oxygen isotopic composition of seawater (d 18 O sw , where d is the deviation from a known reference material in per mil) during the early Paleogene (45-65 Ma). Understanding the response of d 18 O sw to changes in climatic and tectonic boundary conditions is important because records of carbonate d 18 O document changes in hydrology, as well as changes in temperature and global ice-volume. We present results from an early Paleogene configuration of ModelE-R which indicate that spatial gradients of surface ocean d 18 O sw during this period could have been significantly different to those in the modern ocean. The differences inferred from ModelE-R are sufficient to change early Paleogene sea surface temperature estimates derived from primary carbonate d 18 O signatures by more than ±2°C in large areas of the ocean. In the North Atlantic, Indian, and Southern Oceans, the differences in d 18 O sw inferred from our simulation with ModelE-R are in direct contrast with those from another d 18 O-tracing model study which used different, but equally plausible, early Paleogene boundary conditions. The large differences in d 18 O sw between preindustrial and early Paleogene simulations, and between models, emphasizes the sensitivity of d 18 O sw to climatic and tectonic boundary conditions. For this reason, absolute estimates of Eocene/ Paleocene temperature derived from carbonate d 18 O alone are likely to have larger uncertainties than are usually assumed.