Isotopic Composition Of Ocean Water Research Articles

Oxygen isotope analysis of fossil organic matter by secondary ion mass spectrometry

We have developed an analytical procedure for the measurement of oxygen isotope composition of fossil organic matter by secondary ion mass spectrometry (SIMS) at the sub-per mill level, with a spatial resolution of 20–30μm. The oxygen isotope composition of coal and kerogen samples determined by SIMS are on average consistent with the bulk oxygen isotope compositions determined by temperature conversion elemental analysis – isotope ratio mass spectrometry (TC/EA-IRMS), but display large spreads of δ18O of ∼5–10‰, attributed to mixing of remnants of organic compounds with distinct δ18O signatures. Most of the δ18O values obtained on two kerogen residues extracted from the Eocene Clarno and Early Devonian Rhynie continental chert samples and on two immature coal samples range between ∼10‰ and ∼25‰. Based on the average δ18O values of these samples, and on the O isotope composition of water processed by plants that now constitute the Eocene Clarno kerogen, we estimated δ18Owater values ranging between around −11‰ and −1‰, which overall correspond well within the range of O isotope compositions for present-day continental waters. SIMS analyses of cyanobacteria-derived organic matter from the Silurian Zdanow chert sample yielded δ18O values in the range 12–20‰. Based on the O isotope composition measured on recent cyanobacteria from the hypersaline Lake Natron (Tanzania), and on the O isotope composition of the lake waters in which they lived, we propose that δ18O values of cyanobacteria remnants are enriched by about ∼18±2‰ to 22±2‰ relative to coeval waters. This relationship suggests that deep ocean waters in which the Zdanow cyanobacteria lived during Early Silurian times were characterised by δ18O values of around −5±4‰. This study, establishing the feasibility of micro-analysis of Phanerozoic fossil organic matter samples by SIMS, opens the way for future investigations of kerogens preserved in Archean cherts and of the O isotopic composition of ocean water at that period in time.

Oxygen isotopic variations in modern cetacean teeth and bones: implications for ecological, paleoecological, and paleoclimatic studies

The oxygen isotope ratios (δ18O) preserved in marine sediments have been widely used to reconstruct past ocean temperatures. However, there remain significant uncertainties associated with this method, owing to assumptions about the δ18O of ancient seawater which affects the temperature inferred from sediment δ18O records. In this study, oxygen isotope compositions of phosphate in teeth and bones from five different modern cetacean species, including sperm whale, pygmy sperm whale, short-finned pilot whale, killer whale, and Cuvier’s beaked whale, and three fossil whales were determined. The data were used to assess whether the oxygen isotope ratios of biogenic phosphate (δ18Op) from cetaceans are a reliable proxy for the oxygen isotopic composition of ocean water (δ18Ow). The δ18Op values of modern cetaceans range from 15.5‰ to 21.3‰, averaging (19.6‰± 0.8‰) (n = 136). Using a greatly expanded global cetacean δ18Op dataset, the following regression equation is derived for cetaceans: δ18Ow = 0.95317 (±0.03293) δ18Op – 17.971 (±0.605), r = 0.97253. The new equation, when applied to fossil teeth and bones, yielded reasonable estimates of ancient seawater δ18Ow values. Intra-tooth isotopic variations were observed within individual teeth. Among the selected species, the killer whale (O. orca) has the lowest δ18Op values and the largest intra-tooth δ18Op variation, reflecting its habitat preference and migratory behavior. The results show that oxygen isotope analysis of phosphate in cetacean teeth and dense ear bones provides a useful tool for reconstructing the oxygen isotopic composition of seawater and for examining environmental preferences (including migratory behavior) of both modern and ancient whales.

Inter-mineral Mg isotope fractionation during hydrothermal ultramafic rock alteration – Implications for the global Mg-cycle

Both riverine and ocean waters are enriched in 24Mg compared to the homogeneous chondritic Mg isotopic composition of the Earth's mantle requiring a fractionation step that is generally attributed to low temperature continental crust weathering. Here we present new observations that indicate that this 24Mg enrichment of surface waters may originate from Mg isotope fractionation during the hydrothermal alteration of primary silicate minerals. Mineral separates of hydrothermally altered ultramafic rocks were collected from three different localities in Norway. Coexisting olivine and serpentine exhibit invariant Mg isotope ratios suggesting that serpentinization does not fractionate Mg isotopes. In contrast, carbonation results in significant inter-mineral Mg isotope fractionation between the antigorite, magnesite, and talc. The carbonation of the natural samples is constrained by O isotope thermometry at ∼275 °C and hence closes the temperature gap between previous investigations of the natural distribution of Mg isotopes during surface weathering and magmatic processes. The precursor antigorite has an isotopic composition of δ26Mg (DSM-3)=−0.11±0.05‰, whereas the talc is enriched in 26Mg with mean δMg26=0.17±0.08‰ and the magnesite is depleted in 26Mg with mean δMg26=−0.95±0.15‰. As carbonate minerals dissolve faster than silicate minerals, the chemical weathering of carbonated ultramafic and by analogy mafic rocks on the continents will yield isotopically lighter Mg to natural surface waters consistent with field observations. Moreover, the Mg fractionation observed in this study suggests that sub-seafloor hydrothermal carbonation may be a significant contribution to the Mg isotopic composition of ocean water.

Early Permian seasonality from bivalve δ18O and implications for the oxygen isotopic composition of seawater

Research Article| November 01, 2010 Early Permian seasonality from bivalve δ18O and implications for the oxygen isotopic composition of seawater L.C. Ivany; L.C. Ivany * 1Department of Earth Sciences, Syracuse University, Syracuse, New York 13244-1070, USA *E-mail: lcivany@syr.edu. Search for other works by this author on: GSW Google Scholar B. Runnegar B. Runnegar 2Department of Earth and Space Sciences, and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California 90094-1567, USA Search for other works by this author on: GSW Google Scholar Author and Article Information L.C. Ivany * 1Department of Earth Sciences, Syracuse University, Syracuse, New York 13244-1070, USA B. Runnegar 2Department of Earth and Space Sciences, and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California 90094-1567, USA *E-mail: lcivany@syr.edu. Publisher: Geological Society of America Received: 24 Apr 2010 Revision Received: 20 Jun 2010 Accepted: 22 Jun 2010 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2010 Geological Society of America Geology (2010) 38 (11): 1027–1030. https://doi.org/10.1130/G31330.1 Article history Received: 24 Apr 2010 Revision Received: 20 Jun 2010 Accepted: 22 Jun 2010 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation L.C. Ivany, B. Runnegar; Early Permian seasonality from bivalve δ18O and implications for the oxygen isotopic composition of seawater. Geology 2010;; 38 (11): 1027–1030. doi: https://doi.org/10.1130/G31330.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Oxygen isotope values of sequentially microsampled accretionary carbonate from the thick-shelled calcitic bivalve Eurydesma from the Early Permian of southeastern Australia vary seasonally over 6 yr of growth. Paleotemperature constraints derived from coeval glendonite and ice-rafted clasts, in combination with published biome data, indicate near-freezing winter conditions. However, paleotemperatures calculated from oxygen isotope data are too warm unless the δ18O value of seawater approached −4‰. Associated stenohaline brachiopods and lack of covariance with δ13C argue against significantly reduced salinity, and published δ18O data from across Australia suggest that conditions recorded by this bivalve were typical of the Gondwanan high latitudes. The negative δ18O value of seawater implied by these data reflects in part the expected meridional gradient in marine seawater composition related to latitudinal variation in evaporation and precipitation in Permian oceans, but is also consistent with previous suggestions of a secular increase in the oxygen isotopic composition of ocean water through the Phanerozoic. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Modelling the oxygen isotope distribution of ancient seawater using a coupled ocean–atmosphere GCM: Implications for reconstructing early Eocene climate

One of the motivations for studying warm climates of the past such as the early Eocene, is the enhanced understanding this brings of possible future greenhouse conditions. Traditionally, climate information deduced from biological or chemical proxies have been used to “test” computer model simulations of past climatic conditions and hence establish some of the uncertainties associated with model-based predictions. However, extracting climate information from proxies is itself an interpretative process and discrepancies between climate information inferred from different types of proxy undermines the assumption that model-data conflicts automatically mean that the model is inherently flawed. A new approach which both acknowledges and reduces the uncertainties associated with both model and data is required. Although the oxygen isotopic ratio ( δ 18O) preserved in calcareous marine fossils has been used to reconstruct past seawater temperature for several decades, significant uncertainties associated with this method persist. These include assumptions about past seawater δ 18O for which no proxy exists and which is a key control on the temperature inferred from fossil carbonate. Here we present the results of an early Eocene simulation made using a state-of-the-art General Circulation Model (GCM; HadCM3) with CO 2 set at six times pre-industrial values and which has oxygen isotopes incorporated into the full hydrological cycle and hence simulates the δ 18O of past seawater. This allows us to explore the implications of the different seawater δ 18O correction factors commonly used for δ 18O-based temperature reconstruction. It also allows us to focus model-data comparison on δ 18O rather than interpret ocean temperature, an approach that reduces uncertainties in model-data comparison since the effects of both the temperature and the isotopic composition of ocean water on δ 18O of carbonate are accounted for. The good agreement between model and data for both modern and well-preserved early Eocene carbonate increases confidence in climate reconstructions of this time.

Implementing a New Group of TPW Cells as National Standard: Impact on Calibration Services

The definition of the kelvin is based on the triple-point temperature of highly pure water having the isotopic composition of ocean water (more specifically, the isotopic composition is equivalent to that of VSMOW). Belgian national metrology realizes the triple point of water (TPW) as the mean of temperatures measured in three sealed cells. In order to take into account the isotopic composition effect on TPW temperature, the ensemble of cells was replaced in 2006. Three new cells, with isotopic analysis of the contained water, were bought from different manufacturers. The new group of cells was compared to the old TPW national realization in order to quantify the effect of moving towards a new reference. Two different standard thermometers were used in all the cells to take 10 daily measurements on two different ice mantles. The measured resistances were corrected for hydrostatic head, self-heating, and isotopic composition (when available) before calculating the difference. A difference of about 87 μK was found between the old and the new national references. This difference is transferred to customers’ thermometers and cells through calibrations, and the change has to be documented in each new calibration certificate. An additional consequence of the new ensemble cell implementation is the significant reduction in the spread of deviations of individual cells from the mean temperature. The maximum difference between two cells of the ensemble is 96 μK for the old reference cells and 46 μK for the new reference cells corrected for isotopic composition effects.

Corroborating ecological depth preferences of planktonic foraminifera in the tropical Atlantic with the stable oxygen isotope ratios of core top specimens

Past variability in upper ocean thermocline depth is commonly estimated from the abundance of different species of planktonic organisms or the difference in oxygen isotopic composition between two species of planktonic foraminifera, one that lives in the mixed layer and one that lives in or near the thermocline. To test the latter relationships, we measured the oxygen isotopic composition of eight species of planktonic foraminifera (pink and white varieties of Globigerinoides ruber, Globigerinoides sacculifer without the final chamber, Orbulina universa, Pulleniatina obliquiloculata, Globorotalia menardii, Neogloborotalia dutertrei, and Globorotalia tumida) in surface sediment samples from 31 tropical Atlantic deep‐sea sediment cores. Bayesian analysis was used to compare measured oxygen isotopic compositions with their predictions based on modern data sets of annual temperatures and oxygen isotopic composition of ocean water in the upper 500 m at the core sites. Posterior probability densities for predictive model parameters were computed. Probability distributions of calcification depth for analyzed species corroborated their ecological preferences inferred from net tow and sediment trap data. Robustness of the habitat signals in core top specimens suggests that reconstructions of the entire upper ocean temperature profiles, not just their thermocline depth or temperature, might be possible.

Importance of rain evaporation and continental convection in the tropical water cycle

Atmospheric moisture cycling is an important aspect of the Earth's climate system, yet the processes determining atmospheric humidity are poorly understood. For example, direct evaporation of rain contributes significantly to the heat and moisture budgets of clouds, but few observations of these processes are available. Similarly, the relative contributions to atmospheric moisture over land from local evaporation and humidity from oceanic sources are uncertain. Lighter isotopes of water vapour preferentially evaporate whereas heavier isotopes preferentially condense and the isotopic composition of ocean water is known. Here we use this information combined with global measurements of the isotopic composition of tropospheric water vapour from the Tropospheric Emission Spectrometer (TES) aboard the Aura spacecraft, to investigate aspects of the atmospheric hydrological cycle that are not well constrained by observations of precipitation or atmospheric vapour content. Our measurements of the isotopic composition of water vapour near tropical clouds suggest that rainfall evaporation contributes significantly to lower troposphere humidity, with typically 20% and up to 50% of rainfall evaporating near convective clouds. Over the tropical continents the isotopic signature of tropospheric water vapour differs significantly from that of precipitation, suggesting that convection of vapour from both oceanic sources and evapotranspiration are the dominant moisture sources. Our measurements allow an assessment of the intensity of the present hydrological cycle and will help identify any future changes as they occur.

Character of fluids associated with hydrothermal alteration and metamorphism of Palaeoproterozoic submarine volcanic rocks, Baffin Island, Nunavut, Canada

The Bravo Lake Formation on central Baffin Island, Canada formed in a Palaeoproterozoic (∼1.9 Ga) intrabasinal submarine volcanic rift, the rocks of which preserve mineral assemblages formed during submarine hydrothermal alteration, amphibolite facies regional metamorphism, and retrograde metamorphism. Both stable isotope and cation exchange geothermometry of co-existing mineral assemblages indicate preservation of hydrothermal alteration at ∼300 °C, amphibolite facies metamorphism at ∼590 °C, and retrograde metamorphism at ∼300 °C. Fluid isotopic compositions calculated from δ 18O and δD values measured from minerals suggest that three isotopically distinct fluids interacted with the Bravo Lake Formation during its evolution: (1) Palaeoproterozoic seawater infiltration during hydrothermal alteration; (2) regional metamorphic fluids at low water–rock ratios; and (3) retrograde fluids indicating increasing water–rock ratios. Hydrothermal fluids have δ 18O and δD values near 0‰, similar to modern seawater values. The similarity in the isotopic composition of Palaeoproterozoic and modern seawater is consistent with the buffering of the isotopic composition of ocean water by hydrothermal circulation at mid-ocean ridges by similar processes as operate today. The results of this study show that the isotopic composition of prior fluid events can be preserved in amphibolite facies submarine mafic volcanic rocks if there is minimal penetrative structural deformation and dry prograde metamorphism.

Influence of air–sea fluxes on chlorine isotopic composition of ocean water: Implications for constancy in δ37Cl—A statistical inference

The behaviors of chlorine isotopes in relation to air–sea flux variables have been investigated through multivariate statistical analyses (MSA). The MSA technique provides an approach to reduce the data set and was applied to a set of 7 air–sea flux variables to supplement and describe the variation in chlorine isotopic compositions (δ37Cl) of ocean water.The variation in δ37Cl values of surface ocean water from 51 stations in 4 major world oceans—the Pacific, Atlantic, Indian and the Southern Ocean has been observed from −0.76 to +0.74‰ (av. 0.039±0.04‰). The observed δ37Cl values show basic homogeneity and indicate that the air–sea fluxes act differently in different oceanic regions and help to maintain the balance between δ37Cl values of the world oceans. The study showed that it is possible to model the behavior of chlorine isotopes to the extent of 38–73% for different geographical regions. The models offered here are purely statistical in nature; however, the relationships uncovered by these models extend our understanding of the constancy in δ37Cl of ocean water in relation to air–sea flux variables.

Determination of the lithium isotopic composition of planktic foraminifera and its application as a paleo-seawater proxy

To investigate the potential use of Li isotopes in foraminifera as a paleo-lithium seawater proxy, the Li isotopic compositions of planktic foraminifera ( Orbulina universa, Globigerinoides sacculifer, Globorotalia menardii, and Globorotalia truncatulinoides) were determined by thermal ionization mass spectrometry (TIMS), using phosphate as an ion source, and multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). Rigorous cleaning procedures were employed to purify foraminiferal tests of detrital, metal oxide, and organic phases in single species analyses. Data generated using both methods reveal that δ 7Li in planktic foraminifera resemble seawater such that the isotopic fractionation from the precipitation of foraminiferal calcite appears to be minimal. Down-core measurements indicate that the δ 7Li of O. universa has remained constant over the last 44 ka in the North Atlantic, suggesting little change of Li isotopic composition of ocean water during the last glaciation, consistent with the long mean residence time of Li. Results suggest that planktic foraminifera may provide a reliable record of the lithium isotopic composition of ocean water in the past. The first measurements of seawater δ 7Li from the Indian Ocean average 33.0 ± 1.2‰ (2 σ) throughout the water column. This value is within error of values previously reported for the Atlantic and Pacific Oceans as is expected based on the long mean residence time of Li in the ocean. There is no evidence for a significant decrease in Li/Cl ratios, as reported by some workers, with Indian Ocean waters averaging 9.10 ± 0.11 μg/g.

The concentration and isotopic composition of diffusible Nd in fresh and marine waters

Variations in the Nd isotopic composition of ocean water through time, recorded in marine deposits, are suggested to document changes in erosional input and ocean circulation. Thus characterization of the dissolved Nd component in seawater is important. But few attempts have been made to directly measure truly dissolved Nd. Here we report Nd data, obtained using the technique of diffusive gradients in thin-films (DGT) designed to measure in situ, diffusible labile trace metals in aqueous solutions. The method samples free ions and possibly very small complexes. The concentration and isotopic composition of Nd in fresh, brackish and seawater have been determined with DGT, analyzed with thermal ionization mass-spectrometry, and compared with filtered and bulk water samples. Concentrations measured in water samples and with DGT show that the relative amount of diffusible Nd increases with salinity, from being about 10% in the fresh water to 42% in seawater. At each sampled site, the isotopic composition of Nd in the water shows a similar isotopic composition, within errors, with Nd sampled using the DGT method. These results indicate that there is a complete exchange between the particulate/colloidal fraction and the truly dissolved phase. Therefore our findings suggest that solute Nd reflects both the isotopic composition of the marine sediments and the bulk water.

Oxygen isotope variations in phosphate of biogenic apatites, III. Conodonts

The method for measurement of the isotopic composition of oxygen in phosphates has been improved and adapted for analysis of small quantities of apatite, down to 10 mg. This extension enables one to analyze hand-picked conodont samples with an analytical reproducibility better than ±0.5‰ (1σ). 46 samples of conodonts ranging from the Ordovician to the Pennsylvanian of North America were analysed. Some insoluble phosphatic residues, ichthyoliths and inarticulate brachiopods of the same time range were also measured. The range of the δ 18O values of the analysed conodonts is between 15 and 19‰. It shows a general trend of decreasing d 18O with increasing age, from an average value of about 19 in the Pennsylvanian to 17 in the Ordovician. This trend parallels that previously detected in marine phosphorites. For the time range between the Devonian and Pennsylvanian our data are in agreement with independent paleoclimatic information. Specifically, we detect maximum 18O enrichment at the end of the Pennsylvanian, and minimum enrichment at the end of the Devonian. The difference between these two extremes is equivalent to about 10°C (from about 40° to 30°C), assuming constant isotopic composition of ocean water. The success of oxygen isotopic analysis of conodonts raises the possibility of their use in Paleozoic paleo-oceanography in a similar way to foraminifera in the Mesozoic-Cenozoic.

Late miocene—earliest pliocene glaciation in southern Argentina: implications for global ice-sheet history

In South America at latitude 47°S, till was deposited sometime between 7 m.y. and 4.6 m.y. ago, at a time when the local climate was colder than today's. During this same interval glaciers extended to sea level in southeast Alaska, and widespread cooling of the ocean surface in middle latitudes, worldwide marine regression and change in the oxygen isotopic composition of ocean water occurred. From the last three events, major late Miocene expansion of the Antarctic Ice Sheet has been inferred, on the assumption that the history of North Atlantic ice rafting precludes the existence of Northern Hemisphere ice sheets until 3 m.y. ago. This is disputed, first because precipitation in Antarctica would probably have decreased at temperatures below today's, second because the Antarctic Ice Sheet cannot expand appreciably until buildup of Northern Hemisphere ice sheets has lowered sea level, third because virtually no late Miocene sediments are present at the Labrador Sea DSDP sites that are critical to the reconstruction of North Atlantic ice rafting history, and fourth because the scale of late Miocene glaciation in Alaska is at least permissive for simultaneous buildup of ice at similar latitudes further east. If global ice sheet volume at the end of the Miocene was greater than it is today, by an amount that would have lowered sea level by several tens of meters, the excess ice did not accumulate in Antarctica, but in the Northern Hemisphere, chiefly in North America away from the Atlantic coast.

Palaeoclimatic interpretation of stable isotope ratios in molluscan fossils from Middle Pleistocene marine strata, Wanganui, New Zealand

Fossils molluscan shells from the Okehu and Kai-iwi Groups and Recent molluscan shells from New Zealand coastal waters exhibit considerable intraspecific variation in isotopic composition which is probably related to different (infaunal and epifaunal) modes of life. The epifaunal species Chlamys gemmulata proved the most reliable for providing information on changes in temperature or isotopic composition of ocean water during Okehuan time. Analyses of C. gemmulata shells indicate a substantial decrease in temperature or increase in size of ice-caps at about 0.75 Ma, probably in the latter part of the Günz Glaciation, and thereafter minor oscillations with a slight trend towards warming or decreasing ice until about 0.5 Ma (that is, during the Cromer Interglacial).

Hydrogen and oxygen isotope ratios in nodular and bedded cherts

Hydrogen and oxygen isotopic compositions of cherts (δD for hydroxyl hydrogen in the chert, δ 18O for the total oxygen) have been determined for a suite of samples from the central and western United States. When plotted on a δD-δ 18O diagram, Phanerozoic cherts define domains parallel to the meteoric water line which are different for different periods of geologic time. The elongation parallel to the meteoric water line suggests that meteoric waters were involved in the formation of many cherts. The existence of different chert δ-values for different geologic times indicates that once the granular microcrystalline quartz of cherts crystallizes its isotopic composition is preserved with time. An explanation for the change with time of the isotopic composition of cherts involving large changes with time in the isotopic composition of ocean water is unlikely since δ 18O of the ocean would have had to decrease by about 3‰between Carboniferous and Triassic time and then increase about 5%.` from Triassic to Cretaceous time. Such isotopic changes cannot be accounted for by extensive glaciation, sedimentation of hydrous minerals, or input of water from the mantle into the oceans. The variation with time of the chert δ-values can be satisfactorily explained in terms of past climatic temperature fluctuations if the chert-water isotope fractionation with temperature is approximated by 1000 ln α = 3.09 × 10 6 T −2 – 3.29. Crystallization temperatures so inferred suggest that the average climatic temperatures for the central and western U.S. decreased from about 34 to 20°C through the Paleozoic, increased to 35–40°C in the Triassic, and then decreased through the Mesozoic to Tertiary values of about 17°C. A few data for the Precambrian suggest the possibility that Earth surface temperatures may have reached about 52°C at 1.3 b.y. and about 70°C at 3 b.y.