S. R. Managave

Isotope Dendroclimatology: A Review with a Special Emphasis on Tropics

Isotope dendroclimatological investigations have yielded crucial high-resolution climatic data spanning the past few centuries. The success of using stable isotope ratios of oxygen (δ18O) and hydrogen (δD) of tree cellulose for reconstructing past climate depends to a large extent upon the strength of the correlation between δ18O (and δD) of rainfall and the amount of precipitation (in the tropics) or ambient temperature (mid to high latitudes). The usefulness of carbon isotope ratios (δ13C) on the other hand, depends upon the extent to which stomatal conductance is influenced by relative humidity and soil moisture, and the dependence of photosynthetic rate on light intensity. Temperature indirectly affects the δ13C values of tree cellulose through its association with relative humidity and/or irradiance. These conditions put geographical constraints on suitable locations for isotope dendroclimatological investigations.

Trace, Rare-Earth Elements and C, O Isotope Systematics of Carbonate Rocks of Proterozoic Bhima Group, Eastern Dharwar Craton, India: Implications for the Source of Dissolved Components, Redox Condition and Biogeochemical Cycling of Mesoproterozoic Ocean

The Bhima basin is one of a series of Proterozoic basins that overlie the Archean Dharwar craton of South India. In the present study, we have systematically sampled the carbonate rocks from three stratigraphic horizons of Bhima Group and conducted geochemical and C–O isotopic studies in order to understand the source of dissolved components, redox condition and biogeochemical cycling of Mesoproterozoic Ocean. The presence of original microbial texture and Proterozoic marine like δ18O values (−6.38 to −7.17‰) indicate minimum diagenetic alteration. The carbonates have coherent REE + Y patterns and share the essential shale-normalised characteristics of well oxygenated, shallow ambient seawater, such as, (1) uniform heavy REE enrichment (Nd/YbSN = 0.43 ± 0.06), (2) consistent negative Ce anomalies (Ce/Ce* = 0.60 ± 0.05) and (3) superchondritic Y/Ho ratios (38.07 ± 3.17). The detailed geochemical modeling suggests (1) little influence ( 10%) of river/estuarine run-off to the ambient sea water and possibly minor input from oceanic hydrothermal sources. High positive values of δ13C (3.8‰) in the basal Shahabad carbonates indicate burial of a large mass-fraction of isotopically light organic carbon. The gradual up-section decrease to ~1‰ δ13C suggest transgression and mixing of isotopically heavy coastal water (~4‰) with global Dissolved Inorganic Carbon (DIC) reservoir (~0‰). The short term negative δ13C excursion of magnitude ~5‰ at the base is consistent with upwelling of Oxygen Minimum Zone during the transgression event. The wide variability of δ13C (5.15‰, −1.37 to +3.81‰ PDB) in carbonates indicate greater sensitivity of C-isotope system as a consequence of lower buffering capacity and shrinking size of DIC reservoir, which would indicate increased surface oxidation and release of oxygen to the atmosphere.

Paleoclimate of Peninsular India

The limited spatial and temporal coverage of instrumental weather records precludes the knowledge of long-term climatic changes. To infer such changes, recourse is taken to natural archives that serve as climate proxies. The prominent proxies that offer annual to seasonal temporal resolution include annual rings of trees (Ramesh et al., 1985; Ramesh et al., 1986a; Ramesh et al., 1986b; Ramesh et al., 1988; Ramesh et al., 1989; Managave et al., 2010a; Managave et al., 2010b; Sano M. et al., 2010; Managave et al., 2010c; Managave et al., 2010d; Managave et al, 2010e), corals (Chakraborty et al., 1992; Chakraborty et al., 1993a; Chakraborty et al., 1993b; Chakraborty et al., 1993c; Chakraborty et al., 1994; Chakraborty et al., 1997), ice cores (Nijampurkar et al., 1986), speleothems (Yadava et al., 2004) in some cases and varved sediments (Von Rad et al., 1999). Among these, tree-rings have specific advantages: they have a wide geographic distribution, are annually resolved, show a continuous record, and are easily dated by ring-counting. Seasonality in the growth rate of trees driven by seasonality in the climatic factors can result in well-defined annual growth rings in trees. Individual tree-rings faithfully record contemporary climatic signatures and hence provide an opportunity to decipher the variation in climatic parameters for a duration equivalent to the life-span of the tree.