Centre for Earth Sciences (CEaS)

Assessing the potential of stable calcium isotope ratios for paleotemperature reconstruction from studies of laboratory-precipitated inorganic carbonates and modern corals, fish otoliths and foraminifera

Assessing the potential of stable calcium isotope ratios for paleotemperature reconstruction from studies of laboratory-precipitated inorganic carbonates and modern corals, fish otoliths and foraminifera Mondal, Surajit Determining paleo-seawater composition and temperature is critical for reconstructing Earth’s surface conditions through time and has major implications for understanding the origin and evolution of life on Earth. Paleo-seawater composition is inferred from the chemical and iso-topic compositions of biogenic and inorganic chemical precipitates that form out of seawater. Some of the widely used geochemical and isotopic tracers that have been measured in marine carbonates for determining the physical and chemical attributes (e.g., temperature, pH, salinity) of the past oceans include Sr/Ca, Mg/Ca, Li/Ca, Ba/Ca, 18O, 47, 11B, etc. However, each geochemical and isotopic proxy used for determining paleo-seawater temperature has its limi-tations and requires addressing four main issues: (i) inter-species variability in biogenic car-bonates, (ii) variation in past seawater composition, (iii) post-depositional alteration of sam-ples, and (iv) temperature-calibration of the proxy. Hence, an important component of paleo-climate research involves the development of the robust geochemical and isotopic proxies which are minimally affected by factors (e.g., pH, growth rate, vital effect, etc.) other than temperature. Recent improvements in mass spectrometry have resulted in high-precision Ca isotopic measurements (44/40Ca) which allows us to investigate the applicability of 44/40Ca as a paleotemperature proxy. Due to the high abundance of Ca in inorganic and biogenic car-bonates, the 44/40Ca value is expected to be unaffected by the diagenetic alteration of geolog-ical archives which makes the 44/40Ca composition of carbonates a powerful tool for paleocli-mate research. In this thesis, Ca isotopic measurements were performed on various inorganic and biogenic carbonates along with measurements of other geochemical (Sr/Ca, Mg/Ca, Li/Ca, Ba/Ca, B/Ca) and traditional isotopic proxies (18O, 13C) to understand the efficacy of these different viii proxies in reconstructing water temperatures. A novel sample-loading technique, involving a Re-Ta double filament assembly along with the use of tantalum oxide as an activator, was developed for Ca isotopic measurements using thermal ionization mass spectrometry (TIMS). Using this loading technique, the analytical reproducibility for 44/40Ca was better than 0.08‰ (2SD). Elemental ratios in selected biogenic carbonates were measured using a quadrupole inductively coupled plasma mass spectrometer (ICPMS). Using a cold-plasma technique and bracketing using synthetic and international carbonate standards, high precision data were obtained. For example, the external precision was better than ± 0.03 (SD) for Sr/Ca (mmol/mol) and ± 0.025 (SD) for Mg/Ca (mmol/mol). To understand the effect of temperature and the roles of pH and precipitation rate on Ca isotopic composition of inorganic carbonates, calcite precipitation experiments were conducted in a controlled-laboratory environment at different temperatures (5, 10, 20, 30, 40 and 50 °C). At each temperature, the effects of precipitation rate, pH and calcite dissolution rate on Ca isotopic composition of the precipitated carbonate were investigated. It was observed that the calcite dissolution rate is a function of temperature; at low temperatures (5-10 °C), the Ca isotopic fractionation was maximum (44/40Ca = -0.84 to -1.07‰; 44/40Ca = δ44/40Cacrystal - δ44/40Caparent solution) and the associated dissolution rate was low (0.5×10-7 mol/m2/h). In contrast, at relatively high temperatures (40-50 °C), the calculated dissolution rate was higher (25×10-5 mol/m2/h) while the Ca isotopic fractionation was the low (44/40Ca = -0.1 to -0.07). The results from the precipitation experiments suggest that the temperature-controlled variation in 44/40Ca is ~0.02 ‰/°C in inorganic calcites. The 44/40Ca-T relationship proposed in this study (44/40Ca = 0.024 ± 0.003 × T(°C) -1.08 ± 0.11) for inorganically precipitated carbonates can be used to investi-gate significant temperature changes in the geological past. The proposed 44/40Ca-T relation-ship was applied to published Ca isotopic data for cap carbonates to infer that the seawater ix temperature variability was between -17.9 C to 24.5 °C during Neoproterozoic snowball Earth events and these temperature estimates are consistent with published temperature estimates based on climate/ice sheet models. The second part of the thesis focuses on biogenic carbonates. To understand the efficacy of different geochemical and isotopic proxies, multi-elemental (Sr/Ca, Mg/Ca, Li/Ca, Ba/Ca, B/Ca) and isotopic (44/40Ca, 18O, 13C) measurements were performed on the same set of samples drilled from fourteen consecutive low-density and high-density bands in a Porites sp. coral which was collected from the Kavaratti island of the Lakshadweep archipelago in the Arabian Sea. For this coral, the temperature sensitivity of 44/40Ca was determined as ~0.094 ‰/°C. With the present analytical precision and proposed calibration equation, it is proposed that Ca isotopic composition of corals can be used to reconstruct paleotemperatures with a temperature resolution of ~1°C. The temperatures estimated from the geochemical and isotopic analyses were compared with satellite sea-surface temperatures over seven years. The results indicate that a combination of Sr/Ca, Li/Ca and 44/40Ca measurements in corals provide the most accurate sea-surface temperature. A new set of temperature-proxy relationships have been proposed for the Porites sp. in the Arabian Sea. The proposed proxy-SST relationships are: (i) Sr/Ca (mmol/mol) = 10.46 (±0.04) - 0.054 (±0.002) × SST (°C), (ii) Li/Ca (mol/mol) = -0.134 × SST + 9.99 and (iii) 44/40Ca SRM915a (‰) = 0.094 (±0.01) × SST -1.95 (±0.39). To further evaluate the applicability of Ca isotope ratios as a paleotemperature proxy, 44/40Ca was meas-ured in fish otolith samples. Otolith samples are commonly found in the fossil record and are important archives for paleoclimate research. Seasonal growth bands in otolith can yield high-resolution temperature and paleo-water chemistry where other archives like coral or foraminif-era are not available. Elemental ratios (Sr/Ca, Mg/Ca and Ba/Ca) along with 44/40Ca were measured in otolith samples from six different species of fish collected from different x geological locations with a wide range of temperatures from 2 to 25 °C. These samples were previously characterized for their 18O, 13C, and clumped isotope (47) compositions. The temperature sensitivity of 44/40Ca in fish otoliths (~0.02 ‰/°C) was determined for the first time in this thesis. The results suggest that Ca stable isotopes in fish otoliths can be used as a proxy for paleotemperature reconstruction. There is limited information on the temperature dependence of Ca isotopic fractionation in foraminifera. While some studies have reported a strong temperature dependency of 44/40Ca values (0.24 ‰/°C) in G. sacculifer culture samples, other studies have reported negligible temperature sensitivity of 44/40Ca values in foraminifera. The discrepancy between these stud-ies suggests a complex biomineralization pathway in foraminifera as well as the effects of additional factors on the 44/40Ca composition of foraminifera. To further evaluate the temper-ature sensitivity of 44/40Ca in foraminifera, 44/40Ca values were measured in samples which were also characterised for their 18O, 13C, Mg/Ca and Sr/Ca. Paired measurements of isotopic and elemental ratios were performed in benthic (C. wuellerstorfi), thermocline planktic (G. truncatulinoides and G. inflata) and other planktic species (G. ruber, G. bulloides, O universa, G. siphonifera) of foraminifera collected from the North Atlantic Ocean using a sediment trap. Some of these samples were earlier analysed for Mg/Ca and 18O measurements and used for the Mg/Ca thermometry calibration by Anand et al. (2003). In this study, we have analysed selected samples from Anand et al. (2003), showing a wide range in Mg/Ca values (0.99-4.62 mmol/mol), for their 44/40Ca compositions. When the species G. ruber and those with esti-mated habitat temperatures less than 3 °C are excluded, 44/40Ca values show a significant (r2 = 0.76) relationship with temperature [44/40Ca (‰) = 0.35 exp (0.055 × T (°C)], with temper-ature dependency of ~0.055 ‰/°C for 44/40Ca in foraminifera, thereby suggesting the possi-bility of using 44/40Ca in foraminifera as a paleotemperature proxy with a precision of ± 1.5 xi °C. There is a well-defined positive correlation between 44/40Ca and Sr/Ca for a few selected species, suggesting similar temperature dependent Sr partitioning and 44/40Ca fractionation in foraminifera. However, higher Sr/Ca ratio and low 44/40Ca in G. ruber suggests that the com-position of this surface-dwelling species may be affected by precipitation rate. Our study sug-gests that in addition to Mg/Ca, 44/40Ca in foraminifera can be used for accurate paleotemper-ature reconstruction. Overall the results reported in this thesis document the applicability, use-fulness, and limitations of Ca stable isotopes as a paleotemperature proxy.

CO2 Ventilation, Hydrological Cycle over Southern Ocean and Clumped Isotope Thermometry in Biogenic Carbonates

Tue, 20 Feb 2018 00:00:00 GMT

CO2 Ventilation, Hydrological Cycle over Southern Ocean and Clumped Isotope Thermometry in Biogenic Carbonates Prasanna, K The thesis presents observations on the CO2 concentration and carbon isotopes in air CO2 (δ13C) to constrain the inter-annual variability of carbon inventory over the Southern Ocean between the years 2011-2013. Based on the observation, the region of CO2 venting was identified over the Southern Ocean. Further, isotopic characterization allowed inferring about the possible sources of CO2 degassing and contribution from the dissolved inorganic carbon (DIC) that exsolved to generate CO2. It is concluded that the origin CO2 is mainly from the degassing of CO2 available from the dissociation of DIC or organic degradation. Live Foraminiferal samples of Globigerina bulloides from towing were captured, separated and analysed for δ18O and δ13C from various locations across the Southern Ocean between 10°N−60°S. A large similarities in the estimated values (deduced from simultaneous composition of ocean water 18O, δ13C in DIC and temperature i.e. SST under equilibrium condition) and measured δ18O and δ13C values were observed until 40°S from the equator, and hence it was concluded that the calcification depth of G. bulloides is confined to a depth of ~75-200m till 40°S latitude. However, further south (>40oS) disequilibrium from the estimates was detected. A number of possible reasons were cited for the observed disequilibrium such as (1) Deeper depth habitat (2) Partial dissolution (3) Non-equilibrium calcification (4) Oceanic Suess Effect and (5) Genetic Variability. A box model of isotopic mass balance was presented in this study to explain the pattern of enrichment in the 13C values of sea water DIC with latitude (up to about 43°S). The model shows that a steady state of the carbon isotope ratio of water is achieved in a relatively short time of ~5000 days. Rainwater isotope in the open marine condition across the latitudinal transects over Southern Ocean marking zone of precipitation and evaporation is another element of this thesis. A variation with excess lighter isotopes in rainwater was observed in high latitude rain in this study. Observed isotopic depletion is attributed to rainout process over the ocean. The average rainout fraction over the Southern Ocean in the region of zone of precipitation is ~44%, while it drops to ~25% in the zone of evaporation. Second part of the thesis presents a novel method of isotope thermometry which is called “clumped isotope (13C18O16O16O-2 in the calcite structure) thermometry”. A revision in the thermometry equation relating 47 vs T in synthetic carbonates precipitates and otoliths was proposed. The revised calibration was used on fish otoliths from the modern and past environment to estimate the temperatures. Together with the clumped isotope, conventional stable isotopes in the shell carbonates were measured to effectively reconstruct the seasonal fresh water fraction at seasonal time scales.

Constraining the Lithium Seawater Mass and Isotope Budget: Diagenetic Processes Through Marine Pore Waters

Constraining the Lithium Seawater Mass and Isotope Budget: Diagenetic Processes Through Marine Pore Waters Shaikh, Juzer Idris Silicate weathering consumes CO2 and controls cation fluxes to the ocean, thus playing a critical role in modulating long-term seawater chemistry and climate. There are very few markers of seawater chemistry whose value has changed over time as a function of the uplift – weathering – subduction cycle. Lithium, being one such proxy has been extensively utilized as a geochemical tracer whose long-term evolution in seawater is a function of Urey’s tectonic cycle. Marine pore-waters are an excellent archive to study the sedimentary processes affecting seawater chemistry. Thus, marine pore water Li concentration and isotopes are utilized in elucidating numerous under-constrained diagenetic processes such as marine clay formation, clay transformation, carbonate diagenesis, subduction of slab, and clay dewatering. To constrain the above processes through Li isotopes, we have developed a method for separation of Li from matrix elements using a single-step column chromatography technique and precise measurement of Li isotope ratios using inhouse ICP-QQQ. Some of the features of the developed method are high column recovery 101.0 ± 1.2 % (n = 20), low cumulative Li blank (<0.6 pg) and crustal element blanks (<1.5 ng), high Na tolerance (up to 100:1 of Na:Li), low mass requirement (<0.15 ng per analysis), and a sub-permil precision (±0.6 ‰, 2s). Utilizing the above method, we analysed pore water samples from IODP Leg 339 (Mediterranean outflow) and Leg 379 (Amundsen Sea, Southern Ocean) for Li isotopes to deduce the processes occurring at these sites and its implications on Li seawater budget. At the pore water-sediment interface of continental margins, authigenic alumino-silicate clay (smectite) formation also termed as Reverse Weathering, removes Li from seawater/pore water. The reverse weathering process preferentially uptakes 6Li over 7Li. Thus, pore waters are depleted in Li compared to seawater ([Li]PW < [Li]SW), and the pore water Li isotopic composition is more enriched in 7Li than seawater (δ7LiPW > δ7LiSW). This process occurs in shallow sediment depths where smectite is the dominant clay and illitization is not commenced. However, most pore water profiles exhibit higher Li concentrations and lighter isotopic compositions indicating clay transformation process. During clay transformation i.e., smectite to illite transformation, owing at high pressure and temperature (150-200 Celsius) during burial, isotopically light Li is released from the clays. This release of isotopically light Li increases the pore water Li concentration while driving it isotopically light. During sediment subduction, a significant fraction of this clay bound isotopically light Li is released as a part of clay dewatering. This thesis investigates IODP Leg 339 pore water samples with clay dewatering evidence (recorded by δ18O of pore waters) and IODP Leg 379 pore water samples in silica-rich environment with high Li concentration and lighter isotopic composition relative to seawater helps us to constrain Li seawater mass budget further. A preliminary set of equations that govern flux of an element between the marine sediments and seawater following the general diagenetic equation (GDE) is also developed in the present work. These equations incorporate the effects of diffusion, advection and reaction kinetics in the sediments and thus, govern the transfer of Li within the sediment column. Flux calculations for 5 IODP sites and 12 from the literature have been included in the work, and the implications of these flux calculations have been discussed in detail. A thorough development of this model will lead to establishing a mass and isotope budget in seawater which will be applicable across elements and processes. This fundamental study of Li seawater chemistry brings us a step closer in understanding the complex dynamics of ocean systems.

Crustal evolution and tectonic processes of the Madras Block, India

Crustal evolution and tectonic processes of the Madras Block, India Thanooja, P V The Neoarchaean (2.8–2.5 Ga) era in the earth’s history witnessed significant crustal growth related to the amalgamation and dispersion of ancient continents. The processes leading to the formation and destruction of the crust remain enigmatic. Thus, it becomes critical to investigate the Neoarchaean continental crust for understanding these crustal and tectonic processes operated at that time. The crustal blocks which are situated as a transitional zone in between the Southern Granulite Terrane (SGT) and the Dharwar Craton in India, are the well-preserved example of Neoarchaean terranes. The Madras Block is one of the least studied among different crustal blocks in this region. It consists mainly of charnockite, felsic-orthogneiss and meta-monzo-diorite, with a minor occurrence of amphibolite, meta-volcanic, and meta-pelitic rocks. The detailed petrographical, geochemical, and geochronological investigations on these basement rocks pointed out three petrogenetically distinct types (Type-A, -B, and -C) of charnockite from the Madras Block. The Type-A charnockite comprises felsic minerals, minor clinopyroxene and orthopyroxene, abundant biotite, and amphibole. Relative to Type-A charnockite, Type-B, and Type-C, charnockite is rich in garnet and anhydrous ferromagnesium minerals, respectively. Type-A shows a highly depleted HREE pattern ((La/Yb)N = ~36) with a higher degree of chemical fractionation compared to the Type-B ((La/Yb)N ~11) and Type-C ((La/Yb)N ~5). The present results demonstrate that the Type-A charnockite was formed by melting of the subducting oceanic slab and overriding Mesoarchaean proto-crust with a minor input of mantle wedge during 2.79–2.54 Ga. The combined melting of the underplated mantle and overriding Mesoarchean proto-crust along with mixing and homogenisation of Type-A charnockite, during 2.57–2.50 Ga, led to the formation of Type-B and -C charnockite. All the charnockites were subjected to regional metamorphism at ~2.50 Ga, as the prolonged upwelling and magmatism raised the temperature of the entire crust. The felsic-orthogneiss has a highly fractionated REE profile (La/YbN = ~42) resembling Archaean TTG suite of rocks. The meta-monzo-diorite shows a weakly fractionated REE pattern (La/Yb)N = ~17.6) with relatively higher HREE (∑HREE = ~88 ppm) similar to that of Type-B charnockite. The amphibolite which occurs as enclaves, have relatively flat REE pattern, while those occurring as larger volumes (outcrops) have comparatively high LREE. The similar initial 143Nd/144Nd (0.5091–0.5094) and initial 87Sr/86Sr (0.7016–0.7033), along with field and geochemical evidence, suggest that the felsic-orthogneiss and meta-monzo-diorite were formed from the same parental magma (basaltic in affinity) during 2.57–2.51 Ga. The REE geochemical modelling demonstrates a multi-stage magma generation starting from partial melting of a mantle-derived underplated basaltic source, followed by mixing and homogenisation with crustal felsic melt and different rate of continuous fractionation (25-50%) formed ~2.54 Ga felsic and mafic rock in the Madras Block. Thus, the basement rocks of the Madras Block were formed as the result of the repetitive intrusions of basaltic magma into the lower-crust over a long period of time. Consequently, the proto-underplated basaltic magma (~2.62 Ga) be thermally rejuvenated by another pulse of hotter magma and generated ~2.54 Ga felsic-mafic rocks. Synchronous late Neoarchaean magmatic events and associated metamorphism are also identified from the eastern block of the North China Craton (Yishui terrane, Shandong Peninsula), and the north-central Korean Peninsula (Daeijak Island, NW-Gyeonggi Massif). These two terranes and the southern India (Madras Block) demonstrate their close similarities in geological setting, age, petrochemistry, isotopic composition and metamorphic history. Hence, this study proposes that these terranes were once contiguous as part of a Neoarchaean supercontinent.