Seawater δ18O Research Articles

Chert oxygen isotope ratios are driven by Earth's thermal evolution

The 18O/16O ratio of cherts (δ18Ochert) increases nearly monotonically by ~15‰ from the Archean to present. Two end-member explanations have emerged: cooling seawater temperature (TSW) and increasing seawater δ18O (δ18Osw). Yet despite decades of work, there is no consensus, leading some to view the δ18Ochert record as pervasively altered. Here, we demonstrate that cherts are a robust archive of diagenetic temperatures, despite metamorphism and exposure to meteoric fluids, and show that the timing and temperature of quartz precipitation and thus δ18Ochert are determined by the kinetics of silica diagenesis. A diagenetic model shows that δ18Ochert is influenced by heat flow through the sediment column. Heat flow has decreased over time as planetary heat is dissipated, and reasonable Archean-modern heat flow changes account for ~5‰ of the increase in δ18Ochert, obviating the need for extreme TSW or δ18Osw reconstructions. The seawater oxygen isotope budget is also influenced by solid Earth cooling, with a recent reconstruction placing Archean δ18OSW 5 to 10‰ lower than today. Together, this provides an internally consistent view of the δ18Ochert record as driven by solid Earth cooling over billion-year timescales that is compatible with Precambrian glaciations and biological constraints and satisfyingly accounts for the monotonic nature of the δ18Ochert trend.

Eocene-Oligocene southwest Pacific Ocean paleoceanography new insights from foraminifera chemistry (DSDP site 277, Campbell Plateau)

Despite its major role in the Earth’s climate regulation, the evolution of high-latitude ocean dynamics through geological time remains unclear. Around Antarctica, changes in the Southern Ocean (SO) circulation are inferred to be responsible for cooling from the late Eocene and glaciation in the early Oligocene. Here, we present a geochemical study of foraminifera from DSDP Site 277 (Campbell Plateau), to better constrain thermal and redox evolution of the high latitude southwest Pacific Ocean during this time interval. From 56 to 48 Ma, Mg/Ca- and δ18O-paleothermometers indicate high surface and bottom water temperatures (24–26°C and 12–14°C, respectively), while weak negative Ce anomalies indicate poorly oxygenated bottom waters. This is followed by a cooling of ∼4° between 48 and 42 Ma, possibly resulting from a weakening of a proto-EAC (East Australian Current) and concomitant strengthening of a proto-Ross gyre. This paleoceanographic change is associated with better ventilation at Site 277, recorded by an increasing negative Ce anomaly. Once this proto-Ross gyre was fully active, increasing biogenic sedimentation rates and decreasing Subbotina sp. δ13C values indicate enhanced productivity. This resulted in a shoaling of the oxygen penetration in the sediment pile recorded by increasing the foraminiferal U/Ca ratio. The negative Ce anomaly sharply increased two times at ∼35 and ∼31 Ma, indicating enhanced seawater ventilation synchronously with the opening of the Tasmanian and Drake Passage gateways, respectively. The Oligocene glaciation is recorded by a major increase of bottom seawater δ18O during the EOT (Eocene-Oligocene Transition) while Mg/Ca-temperatures remain rather constant. This indicates a significant ice control on the δ18O record.

The pteropod species Heliconoides inflatus as an archive of late Pleistocene to Holocene environmental conditions on the Northwest Shelf of Australia

There is growing interest in the use of pteropods as potential archives of past changes in ocean chemistry. However, pteropods have rarely been used in studies of millennial-scale sedimentary records, especially in shallow-marine environments. This study obtained core data for the last 16 kyr from the Northwest Shelf of Australia (NWS). Changes in the carbonate saturation state were assessed based on the carbon isotope ratios (δ13C) of shells and the Limacina dissolution index (LDX) measured on the planktonic pteropod species Heliconoides inflatus. In addition, the calcification depth of the pteropods was estimated based on oxygen isotope values (δ18O) of pteropod shells and seawater. Our findings indicate that H. inflatus calcifies at a depth of 95–140 m. This confirms that H. inflatus records a shallow-marine signal on the NWS. The δ13C values of the pteropods record a notable decrease in carbonate ion concentrations after 8.5 ka. This decrease is associated with the post-glacial onset of humid conditions on the NWS. The studied pteropod shells are pristine throughout the 16 kyr section and have low LDX values. Therefore, the LDX proxy appears to lack the sensitivity to be applicable in this highly supersaturated, shallow-marine environment. Until this study, proxies derived from H. inflatus have been exclusively utilized in open-marine settings. Our results indicate that the δ13C values of H. inflatus also represent a useful proxy for carbonate ion concentrations in shallow-marine environments.

The universal imprint of oxygen isotopes can track the origins of seafood

AbstractIdentifying the source of seafood is critical for combatting seafood fraud, but current tools are predominantly developed and applied on a species‐specific basis. This study investigates how multiple marine taxa could be geolocated at global scales by exploiting stable oxygen isotope compositions in carbonate biominerals (δ18Obiomin), where we expect to see universally expressed and predictable spatial variation in δ18Obiomin values across taxa. We constructed global ocean isoscapes of predicted δ18Obiomin values specific to fish (otoliths), cephalopod (statoliths) and shellfish (shells), and a fourth combined “universal” isoscape, and evaluated their capacity to derive δ18Obiomin values among known‐origin samples. High correspondence between isoscape‐predicted δ18Obiomin values and a compiled database of measured, georeferenced values (3954 datapoints encompassing 68 species) indicated that this δ18Obiomin approach works effectively, particularly in regions with highly resolved projections of seawater δ18O composition. When compared to taxon‐specific isoscapes, the universal isoscape demonstrated similar accuracy, indicating exciting potential for universal provenance applications. We tested the universal framework via a case study, using machine‐learning models to identify sample origins amongst regions of divergent (Tropical Asia vs Temperate Australasia) and similar (Temperate Asia vs Temperate Australasia) climates and latitudes. Classification accuracy averaged 75.3% between divergent regions, and 66% between similar regions. When endothermic tuna species were excluded from the analysis, the accuracy between divergent regions increased up to 90% between divergent regions. This study presents the first empirical step towards developing universal chemical markers, which have the potential to support a more inclusive and global approach of validating provenance of seafood.

Clumped-isotope-derived climate trends leading up to the end-Cretaceous mass extinction in northwestern Europe

Abstract. Paleotemperature reconstructions of the end-Cretaceous interval document local and global climate trends, some driven by greenhouse gas emissions from Deccan Traps volcanism and associated feedbacks. Here, we present a new clumped-isotope-based paleotemperature record derived from fossil bivalves from the Maastrichtian type region in southeastern Netherlands and northeastern Belgium. Clumped isotope data document a mean temperature of 20.4±3.8 ∘C, consistent with other Maastrichtian temperature estimates, and an average seawater δ18O value of 0.2±0.8 ‰ VSMOW for the region during the latest Cretaceous (67.1–66.0 Ma). A notable temperature increase at ∼66.4 Ma is interpreted to be a regional manifestation of the globally defined Late Maastrichtian Warming Event, linking Deccan Traps volcanic CO2 emissions to climate change in the Maastricht region. Fluctuating seawater δ18O values coinciding with temperature changes suggest alternating influences of warm, salty southern-sourced waters and cooler, fresher northern-sourced waters from the Arctic Ocean. This new paleotemperature record contributes to the understanding of regional and global climate response to large-scale volcanism and ocean circulation changes leading up to a catastrophic mass extinction.

Sea Surface Temperature and Salinity in Lombok Strait Reconstructed From Coral Sr/Ca and δ18O, 1962–2012

Coral geochemical tracers have been used in studies of the paleoclimatology and paleoceanography of the tropics and subtropics. We measured Sr/Ca and oxygen isotope ratios (δ18O) in a coral sample collected from the southern part of Lombok Strait, a significant outlet of the Indonesian Throughflow (ITF) to the Indian Ocean, to reconstruct the historical record of sea surface temperature (SST) and seawater δ18O. Seawater δ18O can be used to approximate sea surface salinity (SSS) because it reflects the balance of evaporation and precipitation. The resulting time series reconstructed SST and SSS, covering the period 1962–2012, shows no clear trend of global warming, although the record includes a large cooling event (~4°C) during 1996–1997. Although neither SST nor SSS shows a systematic relationship with El Niño–Southern Oscillation and Indian Ocean Dipole (IOD), weak but significant correlations are found partly. In addition, the coral data show signals of major IOD and El Niño events in 1994 and 1997, respectively, although climatic trends recorded in the coral are not consistent with those found along the Java-Sumatra coast. To evaluate other influences on the ITF in Lombok Strait, we compared our coral record with coral records from sites in the Java Sea, the southern part of Makassar Strait, and Ombai Strait. During the northwest monsoon (December–January–February), variations in SST and SSS at Lombok Strait site are similar to those at the Java Sea and southern Makassar sites for the period 1962–1995, which suggests that low-salinity water from the Java Sea is carried at least to the southern part of Makassar Strait where it suppresses the ITF upstream from Lombok Strait. However, the SST and SSS records differ at the three sites during the southeast monsoon (June–July–August), indicating that surface conditions in Lombok Strait vary separately from those in the Java Sea. In the longer term, although global warming has been widely identified in the Indonesian Seas, the coral record shows no clear warming trend in the southern part of Lombok Strait, where fluctuations in the ITF may be modulating the distribution of heat in the surface waters of the western Pacific and eastern Indian Ocean.

Reconstructing seawater δ18O and [formula omitted]O values with solid Earth system evolution

There has been extensive debate about how to link sedimentary oxygen isotope records to changes in solid Earth and surface processes. We present a new model of coupled ocean-crust-mantle evolution to investigate the geological processes that are responsible for the evolution of seawater δ18O values. In our model, the rates of low- and high-temperature crustal alteration are constrained by mantle cooling and crustal evolution. This way, we were able to outline the possible path of solid Earth evolution that matches the observed seawater δ18O records. Our results suggest that marine δ18O values may have been as low as −10‰ to −5‰ in the late Archean. The corresponding Δ′17O value may have been 0.015‰±0.01‰ in the late Archean and then decreased with time. The evolution of seawater δ18O is shown to be sufficiently sensitive to the history of continental formation, and our modeling suggests the presence of a considerable amount of continental crust in the early Archean.

Oxygen Isotopic Studies of a Species of Pitar (Hyphantosoma) from Quilon Formation, Kerala, Southwest India: Inferences on Seasonality during the Miocene (late Burdigalian)

Abstract The carbonaceous shale of the Miocene (late Burdigalian) Quilon Formation (Kerala Basin, southwest India) were collected and processed to retrieve the microfaunal component. In the present study, the oxygen isotope analysis of the growth bands of the recovered bivalves Pitar (Hyphantosoma) simonnei and Ostrea sp. (both retrieved from the Quilon Formation) is emphasized to help reconstruct the palaeomonsoonal seasonality. The oxygen-isotope record for the specimens of P.(Hyphantosoma) simonnei and Ostrea sp. exhibits regular, high amplitude δ18O cycles caused by the yearly seasonal temperature and monsoonal variations. Based on the number of δ18O cycles,P. (Hyphantosoma) simonnei and Ostrea sp. have been considered to be approximately 2- and 4-year-old, respectively, at the time of deposition. The calculated palaeo- seawater δ18O values between -0.6 to -1.1‰ [for P. (Hyphantosoma) simonnei] allow us to distinguish between the wet and dry seasons. In addition, the two-component mixing model is suggestive of seasonal freshwater discharge of ~70-80% during wet season and ~20-40% during the dry season into the depositional system. This reconstructed palaeo-seasonality clearly indicates presence of monsoon system in the south west of India during the Miocene (late Burdigalian).

Intra-skeletal variability in phosphate oxygen isotope composition reveals regional heterothermies in marine vertebrates

Abstract. Strategies used by marine vertebrates to regulate their body temperature can result in local variations, and the knowledge of these regional heterothermies is crucial for better understanding the thermophysiologies of extant and extinct organisms. In order to investigate regional heterothermies in vertebrates, we analysed the oxygen isotope composition of phosphatic skeletal elements (δ18Op) of two endothermic fishes (Thunnus thynnus and Xiphias gladius) and three dolphins (two Delphinus delphis delphis and one Cephalorhynchus commersonii kerguelensis). We observed a consistent link between δ18Op variations and temperature heterogeneities recorded by classical methods. Our δ18Op data indicate that: (i) bone hydroxylapatite of the axial skeleton of dolphins mineralise at a warmer temperature than that of the appendicular one, (ii) the skull is the warmest body region in X. gladius, and (iii) T. thynnus possesses high body temperature in the skull and visceral mass region. These results demonstrate the possibility of tracking regional heterothermies in extant marine vertebrates using the δ18Op, paving the way to direct assessment of thermophysiological specificities of both living and extinct vertebrates. From a palaeoenvironmental perspective, the significant observed δ18Op variability questions the use of some taxa or random skeletal elements for the reconstruction of palaeoceanographic parameters such as seawater temperature and δ18O.

An exceptional record of millennial-scale climate variability in the southern Iberian Margin during MIS 6: Impact on the formation of sapropel S6

The study of planktic and benthic δ18O at site U1389 in the Gulf of Cadiz allowed us to reconstruct climate variability during the penultimate glacial period at an unprecedented millennial scale resolution. Hereby, a sequence of interstadial-stadial episodes similar to the Dansgaard-Oeschger events recorded during Marine Isotope Stage (MIS)3, were recognized in MIS6. After a detailed correlation with millennial-scale variability recorded along the Iberian margin and the North Atlantic we were able to link the millennial changes next to the Iberian Peninsula with Antarctic climate variability due to the interhemispheric seesaw response of the Atlantic circulation. The straight coupling at site U1389 of a) the planktic δ18O that reflects climate change in the Atlantic with b) the benthic δ18O, which records the temperature and oxygen isotope composition of the mixture of Mediterranean Outflow and Atlantic intermediate water, indicates that millennial climate variability also had a strong impact on the eastern Mediterranean. The detailed analysis of the benthic δ13C and the fine sand content in the sediments from site U1389 led us to recognize the response of the Mediterranean overflow water (MOW) to millennial scale changes in Mediterranean overturning circulation during the penultimate glaciation. Both Mediterranean Overturning Circulation and MOW strength increased at times of cool and arid climates and weakened during warm and more humid episodes, similar to what has been described for MIS3. The tuning of MOW weakening events occurring at times of precession minima with eastern Mediterranean sapropels and enhanced Asian monsoon allowed the elaboration of a new chronology for MIS6. It was compared with the Epica Dome C time scale, which was also based on the tuning of atmospheric δ18O with Asian Monsoons speleothem records.Mediterranean overturning during MIS6 was also strongly affected by freshwater perturbations occurring at times of insolation maxima, leading to the formation of the “glacial sapropel” S6. Two major drops in seawater δ18O centered at 175 and 150 ky were observed near the entrance of the Mediterranean that were related to meltwater released from the ice sheets, coinciding with periods of insolation maxima. The entry of this meltwater anomaly through the Strait of Gibraltar is clearly registered in the western and eastern Mediterranean, especially at the onset of sapropel S6, suggesting that it was a major component of the freshwater that initiated the buoyancy gain preceding deep water stagnation.

Identifying Hydro‐Sensitive Coral δ18O Records for Improved High‐Resolution Temperature and Salinity Reconstructions

AbstractStable oxygen isotopic ratios in corals (δ18Ocoral) are commonly utilized to reconstruct climate variability beyond the limit of instrumental observations. These measurements provide constraints on past seawater temperature, due to the thermodynamics of isotopic fractionation, but also past salinity, as both salinity and seawater δ18O (δ18Osw) are similarly affected by precipitation/evaporation, advection, and other processes. We use historical observations, isotope‐enabled model simulations, and the PAGES Iso2k database to assess the potential of δ18Ocoral to provide information on past salinity. Using ‘‘pseudocorals’’ to represent δ18Ocoral as a function of observed or simulated temperature and salinity/δ18Osw, we find that δ18Osw contributes up to 89% of δ18Ocoral variability in the Western Pacific Warm Pool. Although uncertainty in the δ18Osw‐salinity relationship influences the inferred salinity variability, corals from these sites could provide valuable δ18Osw reconstructions. Coordinated in situ monitoring of salinity and δ18Osw is vital for improving estimates of hydroclimatic change.

Earth's earliest hydrosphere recorded by the oldest hydrothermally-altered oceanic crust: Triple oxygen and hydrogen isotopes in the 4.3-3.8 Ga Nuvvuagittuq belt, Canada

The origin, evolution, and the state of Earth's hydrosphere are relevant for the timing, style, and intensity of plate tectonics, continental and submarine weathering, and the emergence of the continents. We here present a study of triple oxygen isotopes of hydrothermally altered oceanic crust from the Nuvvuagittuq greenstone belt (NGB) in Canada (4.3-3.8 Ga), which provides a rare snapshot of the Earth's earliest hydrosphere on a planet without modern-style plate tectonics. High δ18O = 8-12‰, Δ′17O=-0.02 to −0.05‰, and δD of −30-40‰ values measured in the NGB metavolcanic rocks are interpreted to indicate that they represent the upper section of ancient oceanic crust (pillows, sediments) altered at low-temperature by seawater similar to modern submarine oceanic sections. Hydrothermal alteration was accompanied by silicification at low temperature (50-150°C) as evidenced by a positive correlation of δ18O with silica, elevated Δ′17O values, and triple oxygen isotope thermometry. We discuss the effects of 2.7 Ga metamorphism on the preservation of triple oxygen isotope compositions and show that the hydrothermal alteration is protolithic and predated metamorphism. Triple oxygen isotopes allow reconstruction of the isotopic compositions of seawater and suggest that the δ18O of the early oceans could have been comparable to that of the modern to perhaps −5‰ but silica-saturated. Models with an ancient ocean with δ18O of +3‰ or <−5‰ seawater result in worse fits to the data. Submarine weathering and hydrothermal silicification could have played a more important role in Eoarchean world with subdued role of subaerial weathering due to insignificant continental exposure that helped balancing seawater δ18O values to near modern values.

Assessing the impact of freshwater discharge on the fluid chemistry in the Svalbard fjords

Changes in the cryosphere extent (e.g., glacier, ice sheet, permafrost, and snow) have been speculated to impact (bio)geochemical interactions and element budgets of seawater and pore fluids in Arctic regions. However, this process has rarely been documented in Arctic fjords, which leads to a poor systematic understanding of land-ocean interactions in such a warming-susceptible region. Here, we present the chemical and isotopic (δ18O, δD, δ11B, and 87Sr/86Sr) compositions of seawater and pore fluids from five fjords in the Svalbard archipelago. Compared to bottom seawater, the low Cl− concentrations and depleted water isotopic signatures (δ18O and δD) of surface seawater and pore fluids delineate freshwater discharge originating from precipitation and/or meltwater of the cryosphere (i.e., glacier, snow, and permafrost). In contrast, the high Cl− concentrations with light water isotopic values in pore fluids from Dicksonfjorden indicate a brine probably resulted from submarine permafrost formation during the late Holocene, a timing supported by the numerical simulation of dissolved Cl− concentration.The freshwater is influenced by the local diagenetic processes such as ion exchanges indicated by δ11B signatures as well as interactions with bedrock during fluid migration inferred from pore fluid 87Sr/86Sr ratios. The interactions with bedrock significantly alter the hydrogeochemical properties of pore fluids in each fjord, yielding spatiotemporal variations. Consequently, land-ocean interactions in combination with the hydrosphere-cryosphere-lithosphere are critical factors for understanding and predicting the hydrology and elemental cycling during global climate change periods in the past, present, and future of the Svalbard archipelago.

Mid to late 20th century freshening of the western tropical South Atlantic triggered by southward migration of the Intertropical Convergence Zone

In the tropical Atlantic Ocean, the Intertropical Convergence Zone (ITCZ) is an important climate feature controlled by the interhemispheric sea surface temperature (SST) gradient, and greatly influences rainfall patterns over the adjacent continents. To better understand ITCZ dynamics in the context of past and future climate change, long-term oceanic records are needed, but observational data are limited in temporal extent. Shallow-water corals provide seasonally-resolved archives of climate variability over the tropical ocean. Here we present seasonally-resolved records of stable oxygen (δ18O) and carbon (δ13C) isotope values of a Siderastrea stellata coral from northeastern Brazil (Maracajaú, ~5°S). We show that the long-term trends in the record of coral δ18O values are not primarily driven by SST but by hydrological changes at the sea surface. Combining the record of coral δ18O values with instrumental SST, we present the first reconstruction of seawater δ18O changes (δ18Oseawater) in the western tropical South Atlantic back to the early 20th century, a parameter that is related to changes in sea surface salinity. The reconstructed δ18Oseawater changes indicate a prominent freshening between the mid-1940's and mid-1970's, which coincides with a weakening of the Atlantic interhemispheric SST gradient during this time interval. Our results suggest that the weakened Atlantic SST gradient resulted in a southward shift of the thermal equator that was accompanied by a southward migration of the ITCZ, resulting in freshening of the western tropical South Atlantic during the mid to late 20th century.

Reassessment of ocean paleotemperatures during the Late Ordovician

Abstract The Late Ordovician is a paradox, with a greenhouse climate shifting to an icehouse climate during a time of presumably high CO2 levels. We used clumped isotope (Δ47) microanalysis of fossil brachiopod shells to determine paleotemperatures of North America (Cincinnati Arch) during the Katian (453–443 Ma). Microanalyses of fossil brachiopods yielded a right-skewed distribution of clumped isotope temperatures [T(Δ47)] ranging from 25 °C to 55 °C with a mode of 32 °C and a mean of 36 °C. To test the impact of Δ47 reordering (reequilibration of 13C-18O bonding) at burial temperatures on T(Δ47), we applied a Monte Carlo simulation to a Δ47 reordering model based on the burial history. Because the reordering simulation indicated partial reordering, we used the simulation results to back-calculate T(Δ47) and correct the reordering effect. Correcting for reordering decreased the mean T(Δ47) by only ~1 °C, to 35 °C, and did not change the mode (32 °C). These temperatures are lower than those of previous studies, but still suggest that the Late Ordovician climate of the Cincinnati Arch was warmer than the modern subtropical seas. However, carbonate facies in these strata argue for upwelling of cool water; thus, typical subtropical surface waters may have been as warm as 35 °C approaching the Hirnantian glaciation. Seawater δ18O values derived from back-calculated T(Δ47) and brachiopod δ18O values average −0.3‰ ± 0.6‰ relative to Vienna standard mean ocean water (VSMOW), consistent with the value expected for subtropical surface waters in a greenhouse Earth, assuming a constant seawater-δ18O model.

The spatial hydrological structure of the western tropical Pacific derived from δ18O and δD signatures

Lack the connection between the air-sea interaction and deep hydrological processes hinders to clarify the response of Western Pacific warm pool (WPWP) to recent global warming. In this paper, δ18O, δD, combined with other properties of seawater collecting during the 37th and 47th Chinese Research Expedition cruises, were measured to trace the potential hydrological processes in the tropical Pacific Ocean. The change in δ18O and d-excess signatures of surface water revealed that two different zones were separated at 155° E along the transect. This coincided with the spatial evaporation–precipitation pattern of the Pacific Walker circulation (PWC). At station CTD03, 18O-enriched water (salinity ∼ 35 psu, δ18O ∼ 0.20‰ in average) at a depth of 100 ∼ 150 m was potentially attributed to the recirculation of the Kuroshio Extension Flow (KEF). In contrast, the deep water with low δ18O (− 0.60‰ in average from 2000 m to 5680 m) indicated the dominant Lower Circumpolar Pacific Water (LCPW). These contributions of water mass sources to the hydrological structure of western Pacific warm pool (WPWP) were confirmed by a mass balance model. Furthermore, significant linear correlations between chlorophyll and δ18O, as well as chlorophyll and d-excess within the upper 200 m at station CTD03, indicated that the seawater δ18O signature could be impacted by biogenic fractionation. The research highlights the validity of δ18O and d-excess to indicate both hydrological processes and watermass structure in WPWP. Further proofs based on investigation and experiments are required.

Microstructural Mapping of Arctica islandica Shells Reveals Environmental and Physiological Controls on Biomineral Size

The shells of long-lived bivalves record environmental variability in their geochemical signatures and are thus used extensively in marine high-resolution paleoclimate studies. To possibly overcome the limitations of the commonly employed temperature proxy, the δ18Oshell value, which requires knowledge of the seawater δ18O signature and is prone to diagenetic overprint, the shell microstructures and the morphological properties of individual biomineral units (BMUs) recently attracted research interest as an alternative paleoclimate proxy. In shells of A. islandica, one of the most extensively used and best studied sclerochronological archives, the size of the BMUs increases in warmer temperatures under laboratory circumstances. This study assesses whether this relationship persists under natural growth conditions or whether additional environmental and physiological factors control the BMU size and bias temperature reconstructions. For this purpose, shells from the surface waters of NE Iceland and the Baltic Sea, as well as from deeper waters of the North Sea (100 and 243 m) were analyzed by means of SEM. The BMU sizes were measured by means of image processing software. Results demonstrate a strong effect of temperature on the BMU size at NE Iceland and in the North Sea at 100 m depth. At 243 m depth, however, temperature variability was likely too low (1.2°C) to evoke a microstructural change. At the Baltic Sea, the BMUs remained small, possibly due to physiological stress induced by low salinity and/or hypoxia. Thus, the size of BMUs of A. islandica shells only serves as a relative temperature indicator in fully marine habitats, as long as seasonal temperature amplitudes exceed ca. 1°C. Furthermore, BMU size varied through lifetime with the largest units occurring during age seven to nine. This pattern is possibly linked to the shell growth rate or to the amount of metabolic energy invested in shell growth.

Using Stable Isotopes (δ18O and δD) to Study the Dynamics of Upwelling and Other Oceanic Processes in Northwestern South China Sea

AbstractFew systematic investigations use isotopic compositions (δ18O and δD) of seawater together with water temperature and salinity to trace the water dynamic processes in the upwelling area. This study presents depth‐specific δ18O, δD, salinity (S), and temperature data of water samples collected from the northwestern South China Sea (NWSCS) during spring and summer 2018 to study upwelling and other oceanic physical processes. Strong upwelling is observed in the southwestern area of the NWSCS in both spring and summer based on the distribution of water temperature. In spring, the area with lower water temperature has generally higher δD and δ18O values than other studied areas, while this phenomenon is not observed in summer. This indicates different hydrodynamic conditions in these seasons. The eastward advection of upwelled water in summer contributes to this phenomenon, which can be supported by the δ18O‐S relationships. δ18O‐S regression line in summer has higher slope than that in spring. Five water masses are defined in both seasons. In the west area of the NWSCS, the offshore upper water (OUW) is mainly formed by the mixing of coastal water and offshore deep water (ODW). We use the principle of mass conservation to estimate the relative contributions of ODW based on δ18O. ODW in spring contributes 40% to OUW, while in summer it contributes 69%. This study presents the method of using stable isotopes to study the upwelling event in NWSCS and quantifies the contribution of upwelling which obtains more information compared with the traditional hydrological observations.

Coral Oxygen Isotopic Records Capture the 2015/2016 El Niño Event in the Central Equatorial Pacific

AbstractCoral oxygen isotopes (δ18O) from the central equatorial Pacific provide monthly resolved records of El Niño‐Southern Oscillation activity over past centuries to millennia. However, calibration studies using in situ data to assess the relative contributions of warming and freshening to coral δ18O records are exceedingly rare. Furthermore, the fidelity of coral δ18O records under the most severe thermal stress events is difficult to assess. Here, we present six coral δ18O records and in situ temperature, salinity, and seawater δ18O data from Kiritimati Island (2°N, 157°W) spanning the very strong 2015/16 El Niño event. Local sea surface temperature (SST) anomalies of +2.4 ± 0.4°C and seawater δ18O anomalies of −0.19 ± 0.02‰ contribute to the observed coral δ18O anomalies of −0.58 ± 0.05‰, consistent with a ∼70% contribution from SST and ∼30% from seawater δ18O. Our results demonstrate that Kiritimati coral δ18O records can provide reliable reconstructions even during the largest class of El Niño events.

Triple oxygen isotope systematics of diagenetic recrystallization of diatom opal-A to opal-CT to microquartz in deep sea sediments

The oxygen-18 isotopic composition (δ18O) of silica preserved in oceanic sediments is an important archive of Earth’s temperature and/or seawater δ18O from the Archean to present. Recent advances in high-precision measurements of both δ18O and δ17O values have been used to provide additional constraints on what the oxygen isotopic composition of chert reflects about past conditions. Here, we examine the effects on the triple oxygen isotopic composition of chert that occurs during transformation and recrystallization of biogenic opal-A to opal-CT to microquartz in deep sea sediments. We studied late Miocene to present samples from the Sea of Japan at ODP Site 795 and measured biogenic diatom opal-A, opal-CT, microquartz chert, and ‘altered’ opal-A samples—previously measured for δ18O values only—for both δ18O and δ17O values. We find that δ18O decreases and Δ′17O increases (where Δ′17O = δ17O – 0.528 × δ18O) with depth, coincident with the conversions from diatom opal-A to opal-CT to microquartz. Silica samples deviate from the trend expected for triple oxygen isotopic equilibrium with modern seawater. To explain these data, we developed a model that shows that local temperature gradients and pore fluid δ18O profiles in combination lower the measured opal-CT and microquartz δ18O values and raise the Δ′17O values relative to the initial opal-A, but deviate from triple oxygen isotopic equilibrium with seawater. We find that a steeper local temperature gradient and a larger influence of hydrothermal alteration of basalt at the base of the sediment column (which lowers pore fluid δ18O values) in the past explain both the measured δ18O and Δ′17O values of the opal-CT and microquartz.These data and our modeling show that the transformation of opal-A to microquartz in marine sediments at elevated temperatures and in the presence of lowered pore water δ18O values leads to an array that falls below the theoretical triple oxygen isotope line of equilibrium for SiO2 with modern seawater. Further, our model indicates that opal-CT and microquartz do form in triple oxygen isotopic equilibrium with pore fluids that are offset in their triple oxygen isotopic composition compared to seawater due to fluid-rock alteration of igneous rocks at the base of the sediment column. The diagenetic processes taking place in the Japan Sea do not explain a large portion of the existing Archean to present triple oxygen isotope chert data, which likely require either changes in the oxygen isotopic composition of the source water (i.e., ocean water) and/or alteration by meteoric fluids. Further, our data demonstrate that the triple oxygen isotopic composition of preserved chert need not represent surface conditions, but instead may reflect processes that occur in subsurface sediments at elevated temperatures and with modified pore fluid oxygen isotopic compositions.