Oxygen Isotopic Composition Of Seawater Research Articles

Machine Learning Solutions to Regional Surface Ocean δ18O‐Salinity Relationships for Paleoclimatic Reconstruction

AbstractStable isotope‐based reconstructions of past ocean salinity and hydroclimate depend on accurate, regionally constrained relationships between the stable oxygen isotopic composition of seawater (δ18Osw) and salinity in the surface ocean. An increasing number of δ18Osw observations suggest greater spatial variability in this relationship than previously considered, highlighting the need to reassess these relationships on a global scale. Here, we use available, paired δ18Osw and salinity data (N = 11,119) to create global interpolations of each variable. We then use a self‐organizing map, a specialized form of machine learning, to define 19 regions with unique δ18Osw‐salinity relationships in the surface (<50 m) ocean. Inclusion of atmospheric moisture‐related variables and oceanic tracer data in additional self‐organizing map experiments indicates global surface δ18Osw‐salinity spatial patterns are strongly forced by the atmosphere, as the SOM spatial output is highly similar despite no overlapping input data. Our approach is a useful update to the previously delimited regions, and highlights the utility of neural network pattern extraction in spatiotemporally sparse data sets.

The CoralHydro2k database: a global, actively curated compilation of coral δ18O and Sr ∕ Ca proxy records of tropical ocean hydrology and temperature for the Common Era

Abstract. The response of the hydrological cycle to anthropogenic climate change, especially across the tropical oceans, remains poorly understood due to the scarcity of long instrumental temperature and hydrological records. Massive shallow-water corals are ideally suited to reconstructing past oceanic variability as they are widely distributed across the tropics, rapidly deposit calcium carbonate skeletons that continuously record ambient environmental conditions, and can be sampled at monthly to annual resolution. Climate reconstructions based on corals primarily use the stable oxygen isotope composition (δ18O), which acts as a proxy for sea surface temperature (SST), and the oxygen isotope composition of seawater (δ18Osw), a measure of hydrological variability. Increasingly, coral δ18O time series are paired with time series of strontium-to-calcium ratios (Sr/Ca), a proxy for SST, from the same coral to quantify temperature and δ18Osw variability through time. To increase the utility of such reconstructions, we present the CoralHydro2k database, a compilation of published, peer-reviewed coral Sr/Ca and δ18O records from the Common Era (CE). The database contains 54 paired Sr/Ca–δ18O records and 125 unpaired Sr/Ca or δ18O records, with 88 % of these records providing data coverage from 1800 CE to the present. A quality-controlled set of metadata with standardized vocabulary and units accompanies each record, informing the use of the database. The CoralHydro2k database tracks large-scale temperature and hydrological variability. As such, it is well-suited for investigations of past climate variability, comparisons with climate model simulations including isotope-enabled models, and application in paleodata-assimilation projects. The CoralHydro2k database is available in Linked Paleo Data (LiPD) format with serializations in MATLAB, R, and Python and can be downloaded from the NOAA National Center for Environmental Information's Paleoclimate Data Archive at https://doi.org/10.25921/yp94-v135 (Walter et al., 2022).

Cenozoic evolution of deep ocean temperature from clumped isotope thermometry.

Characterizing past climate states is crucial for understanding the future consequences of ongoing greenhouse gas emissions. Here, we revisit the benchmark time series for deep ocean temperature across the past 65 million years using clumped isotope thermometry. Our temperature estimates from the deep Atlantic Ocean are overall much warmer compared with oxygen isotope-based reconstructions, highlighting the likely influence of changes in deep ocean pH and/or seawater oxygen isotope composition on classical oxygen isotope records of the Cenozoic. In addition, our data reveal previously unrecognized large swings in deep ocean temperature during early Eocene acute greenhouse warmth. Our results call for a reassessment of the Cenozoic history of ocean temperatures to achieve a more accurate understanding of the nature of climatic responses to tectonic events and variable greenhouse forcing.

Disentangling the Impact of Global and Regional Climate Changes During the Middle Eocene in the Hampshire Basin: New Insights From Carbonate Clumped Isotopes and Ostracod Assemblages

AbstractWe present a novel multiproxy data set (bulk and clumped isotopes on gastropod shells and variations in ostracod assemblages) of the Hampshire Basin (Southern‐England) that sheds light on the connection between the North Sea and the Atlantic Ocean through the English Channel during the deposition of the Barton clay formation (latest Lutetian—Bartonian, middle Eocene; ∼41–40 Ma). During this time period, the English Channel operated as a gateway between the warmer Atlantic Ocean and the colder North Sea waters. High‐latitude water mixing combined with the regional hydrological cycle and sea‐level variations, may have contributed to mitigate the water temperatures in the Hampshire Basin, with an average recorded of ca. 25°C. In the uppermost part of the section the connection between the two water masses was limited or absent as evidenced by warmer (up to ca. 35°C) Atlantic Ocean temperatures in the Hampshire Basin. The large differences in average oxygen isotope composition of seawater (δ18Owater) recorded (from δ18Owater −1.7‰ to δ18Owater of −0.7‰) evidenced large salinity differences between the Atlantic Ocean and the North Sea. Ostracods suggest that the temporary connection between those two water masses was caused by relative regional sea‐level variations. This scenario could be partially linked to glacio‐eustatic sea‐level changes related to climate fluctuations probably associated with the MECO event.

Oxygen isotope composition of seawater and salinity in the western Indian Ocean: Implications for water mass mixing

In this study, the quasi-meridional distribution of oxygen isotopes in seawater (δ18Osw) and salinity were investigated to understand water mass mixing in the western Indian Ocean along the two transects 60°E and 67°E, and covering latitudinal ranges from 3°S to 12°S and 3°S to 25°S, respectively. We identified the water mass mixing using the relationship between δ18Osw and salinity newly presented herein. Results show three linear mixing lines divided into three water layers on the δ18Osw salinity diagram. A positive correlation associated with evaporation and precipitation was detected in the surface layer (<100 m), while in the intermediate (100–1000 m) and deep (>1000 m) layers, distinctive vertical mixing lines depending on the latitude were observed. Vertical mixing trends were discerned in the northern (Arabian Sea Surface Water [ASSW], Indian Deep Water [IDW], and Circumpolar Deep Water [CDW]), and southern (South Indian Subtropical Underwater [STUW], Antarctic Intermediate Water [AAIW], IDW, and CDW) areas of the study region. Based on a simple mixing model of conservative behaviors for δ18Osw and salinity, we estimated the contribution of each of the five water masses in the western Indian Ocean. ASSW and STUW overlying the intermediate layer showed the highest contributions in the northern and southern areas of study region, respectively. In the intermediate layer, AAIW had the highest fraction in the southern area while IDW generally occupied the intermediate layer overlying CDW from north to south. Our estimations were similar to the results of published studies obtained in the western Indian Ocean, and the water mass fractions obtained in this study well reflected the presence of major water masses depending on latitude and depth.

Seawater chemistry of a modern subtropical ‘epeiric’ sea: Spatial variability and effects of organic decomposition

Direct studies of recent epeiric seawater chemistry and their geological significance are relatively few compared to the far more abundant studies from ancient epeiric marine carbonates. Acknowledging the limitations of analogue studies in the modern glaciated world, we propose that the recent Abu Dhabi offshore and lagoonal areas share important attributes with the shallow-marine coastal zones of a subtropical epeiric carbonate sea. The study area in Abu Dhabi is dwarfed by the vast epeiric seas of the geological past, but its seawater properties and related geochemistry have relevance for paleoenvironmental interpretations for similar ancient systems. This study documents the geochemical properties of seawater along the Abu Dhabi coast, and discusses their trends within a spatial context. Results show a positive correlation between salinity, temperature, elemental concentrations, and oxygen isotopic composition of seawater, indicating a significant influence of evaporation on seawater chemistry. A distal-to-proximal negative correlation between seawater Mg/Ca and Sr/Ca ratios may be related to a landward increase in abundance of high-Mg calcite foraminifera. After normalization to chloride, seawater elemental (Ca2+, Mg2+, Sr2+) concentrations and alkalinity show decreasing trends in a landward direction, consistent with seawater aging from the open marine to restricted lagoonal settings. Lower ΔAlk (alkalinity change) : ΔCa2+ (Ca2+ change) ratios in seawater compared with the theoretical value, and more negative δ13CDIC values relative to typical (sub)tropical oceans in the study area indicate an influence of factors apart from CaCO3 precipitation on properties of aged seawater. An input of alkalinity and 12C-enriched carbon by organic carbon decomposition (via sulfate reduction) during early diagenesis is proposed to account for these deviations from typical values. The input of alkalinity and 12C-enriched carbon by organic carbon decomposition may be further related to reduced seawater pCO2 and early-marine cementation. Findings discussed here suggest that organic carbon decomposition during early-marine diagenesis may influence the geochemical properties of the coastal areas of shallow epeiric seas, and thus assumedly produce significant differences in proxy data compared with those from carbonates deposited in open oceans.

Holocene Evolution of Sea‐Surface Temperature and Salinity in the Gulf of Mexico

AbstractFlows into and out of the Gulf of Mexico (GoM) are integral to North Atlantic Ocean circulation and help facilitate poleward heat transport in the Western Hemisphere. The GoM also serves as a key source of moisture for most of North America. Modern patterns of sea‐surface temperature (SST) and salinity in the GoM are influenced by the Loop Current, its eddy‐shedding dynamics, and the ensuing interplay with coastal processes. Here, we present sub‐centennial‐scale records of SST and stable oxygen isotope composition of seawater (18Osw; a proxy for salinity) over the past 11,700 years using planktic foraminiferal geochemistry in sediments from the Garrison Basin, northwestern GoM. We measured O and magnesium‐to‐calcium ratios in tests of Globigerinoides ruber (white) to generate quantitative estimates of past sea‐surface conditions. Our results replicate and extend late Holocene reconstructions from the Garrison Basin, using which we then create composites of SST and 18Osw. We find considerable centennial and millennial‐scale variability in both SST and 18Osw, although their evolution over the Holocene is distinct. Whereas mean‐annual SSTs display pronounced millennial‐scale variability, 18Osw exhibits a secular trend spanning multiple millennia and points to increasing northwestern GoM surface salinity since the early Holocene. We then synthesize the available Holocene records from across the GoM and alongside the Garrison Basin composite uncover substantial, yet regionally consistent, spatiotemporal variability. Finally, we discuss the role of the Loop Current and coastal influx of freshwater in imposing these heterogeneities. We conclude that dynamic surface‐ocean changes occurred across the GoM over the Holocene.

Absolute seasonal temperature estimates from clumped isotopes in bivalve shells suggest warm and variable greenhouse climate

Seasonal variability in sea surface temperatures plays a fundamental role in climate dynamics and species distribution. Seasonal bias can also severely compromise the accuracy of mean annual temperature reconstructions. It is therefore essential to better understand seasonal variability in climates of the past. Many reconstructions of climate in deep time neglect this issue and rely on controversial assumptions, such as estimates of sea water oxygen isotope composition. Here we present absolute seasonal temperature reconstructions based on clumped isotope measurements in bivalve shells which, critically, do not rely on these assumptions. We reconstruct highly precise monthly sea surface temperatures at around 50 °N latitude from individual oyster and rudist shells of the Campanian greenhouse period about 78 million years ago, when the seasonal range at 50 °N comprised 15 to 27 °C. In agreement with fully coupled climate model simulations, we find that greenhouse climates outside the tropics were warmer and more seasonal than previously thought. We conclude that seasonal bias and assumptions about seawater composition can distort temperature reconstructions and our understanding of past greenhouse climates.

A CO2 greenhouse efficiently warmed the early Earth and decreased seawater 18O/16O before the onset of plate tectonics

The low 18O/16O stable isotope ratios (δ18O) of ancient chemical sediments imply ∼70 °C Archean oceans if the oxygen isotopic composition of seawater (sw) was similar to modern values. Models suggesting lower δ18Osw of Archean seawater due to intense continental weathering and/or low degrees of hydrothermal alteration are inconsistent with the triple oxygen isotope composition (Δ'17O) of Precambrian cherts. We show that high CO2 sequestration fluxes into the oceanic crust, associated with extensive silicification, lowered the δ18Osw of seawater on the early Earth without affecting the Δ'17O. Hence, the controversial long-term trend of increasing δ18O in chemical sediments over Earth's history partly reflects increasing δ18Osw due to decreasing atmospheric pCO2 We suggest that δ18Osw increased from about -5‰ at 3.2 Ga to a new steady-state value close to -2‰ at 2.6 Ga, coinciding with a profound drop in pCO2 that has been suggested for this time interval. Using the moderately low δ18Osw values, a warm but not hot climate can be inferred from the δ18O of the most pristine chemical sediments. Our results are most consistent with a model in which the "faint young Sun" was efficiently counterbalanced by a high-pCO2 greenhouse atmosphere before 3 Ga.

Uncertainty in palaeosalinity estimates from foraminiferal geochemical records in the northern Indian Ocean

Palaeosalinity (S*) estimates rely on calculations stemming from the oxygen isotopic composition of foraminiferal shells (δ18OF) preserved in ocean sediments. δ18OF depends on palaeo in-situ temperature (T*) and the oxygen isotopic composition of seawater (δ18Osw), which in turn depends on various physical oceanographic conditions including evaporation, precipitation, advection, and freshwater fluxes. δ18Osw and salinity covary linearly owing to similar controls, as they both increase with increases in evaporation and decrease with higher precipitation and continental runoff. Typically, palaeosalinity is estimated assuming a constant spatial and temporal relationship between δ18Osw and salinity. However, evaporative and freshwater fluxes exhibit spatiotemporal variability and thus, can change the slope and intercept of the linear δ18O-salinity relationship. Hence, the use of a constant δ18Osw-salinity relationship may produce (systematic) errors in palaeosalinity estimates. Here, we revisit palaeosalinity estimates and provide a reassessment of errors arising from these calculations based on recently reported δ18Osw-salinity relationships in the northern Indian Ocean. Our calculations point to errors in palaeosalinity that can be as large as 55% - significant enough to change our understanding of past ocean circulation and palaeomonsoon variability. These results have important implications for reconstructing the palaeoceanographic record and interpreting past ocean-atmosphere variability.

Shell chemistry of the boreal Campanian bivalve &amp;lt;i&amp;gt;Rastellum diluvianum&amp;lt;/i&amp;gt; (Linnaeus, 1767) reveals temperature seasonality, growth rates and life cycle of an extinct Cretaceous oyster

Abstract. The Campanian age (Late Cretaceous) is characterized by a warm greenhouse climate with limited land-ice volume. This makes this period an ideal target for studying climate dynamics during greenhouse periods, which are essential for predictions of future climate change due to anthropogenic greenhouse gas emissions. Well-preserved fossil shells from the Campanian (±78 Ma) high mid-latitude (50∘ N) coastal faunas of the Kristianstad Basin (southern Sweden) offer a unique snapshot of short-term climate and environmental variability, which complements existing long-term climate reconstructions. In this study, we apply a combination of high-resolution spatially resolved trace element analyses (micro-X-ray fluorescence – µXRF – and laser ablation inductively coupled plasma mass spectrometry – LA-ICP-MS), stable isotope analyses (IRMS) and growth modeling to study short-term (seasonal) variations recorded in the oyster species Rastellum diluvianum from the Ivö Klack locality. Geochemical records through 12 specimens shed light on the influence of specimen-specific and ontogenetic effects on the expression of seasonal variations in shell chemistry and allow disentangling vital effects from environmental influences in an effort to refine paleoseasonality reconstructions of Late Cretaceous greenhouse climates. Growth models based on stable oxygen isotope records yield information on the mode of life, circadian rhythm and reproductive cycle of these extinct oysters. This multi-proxy study reveals that mean annual temperatures in the Campanian higher mid-latitudes were 17 to 19 ∘C, with winter minima of ∼13 ∘C and summer maxima of 26 ∘C, assuming a Late Cretaceous seawater oxygen isotope composition of −1 ‰ VSMOW (Vienna standard mean ocean water). These results yield smaller latitudinal differences in temperature seasonality in the Campanian compared to today. Latitudinal temperature gradients were similar to the present, contrasting with previous notions of “equable climate” during the Late Cretaceous. Our results also demonstrate that species-specific differences and uncertainties in the composition of Late Cretaceous seawater prevent trace element proxies (Mg∕Ca, Sr∕Ca, Mg∕Li and Sr∕Li) from being used as reliable temperature proxies for fossil oyster shells. However, trace element profiles can serve as a quick tool for diagenesis screening and investigating seasonal growth patterns in ancient shells.

Carbonate clumped isotope evidence for latitudinal seawater temperature gradients and the oxygen isotope composition of Early Cretaceous seas

In this study, we investigated Early Cretaceous (Valanginian, ca. 135 million years ago) climate from subtropical to boreal palaeolatitudes. Combined carbonate clumped isotope and oxygen isotope data derived from sub-arctic, boreal, and sub-tropical fossil belemnite rostra (Mollusca: Cephalopoda) provide new palaeotemperature estimates as well as a constraint on the oxygen isotope composition of seawater. Our belemnite data reveal balmy high-latitude marine temperatures (ca. 22 °C) and warm sub-tropical temperatures (ca. 31 °C). Supplementing our clumped isotope-based temperature estimates with published TEX86 data results in a conservative reconstruction of a latitudinal temperature gradient that is reduced compared to modern conditions. We find that modelling efforts are close to reproducing tropical temperatures when high pCO2 levels are considered. Warm polar temperatures imply, however, that data-model discrepancies remain. Early Cretaceous seawater oxygen isotope values show a modern profile and are much more positive (up to 1.5‰ SMOW) than typically assumed. Based on our findings, if the positive Cretaceous seawater δ18O values are not considered, carbonate δ18O thermometry would underestimate temperatures, most acute at middle and tropical latitudes.

Corals reveal ENSO-driven synchrony of climate impacts on both terrestrial and marine ecosystems in northern Borneo

Extreme climate events, such as the El Niños in 1997/1998 and 2015/16, have led to considerable forest loss in the Southeast Asian region following unprecedented drought and wildfires. In Borneo, the effects of extreme climate events have been exacerbated by rapid urbanization, accelerated deforestation and soil erosion since the 1980s. However, studies quantifying the impact of interannual and long-term (>3 decades) climatic and anthropogenic change affecting Borneo’s coastal and coral reef environments are lacking. Here, we used coral cores collected in Miri-Sibuti Coral Reefs National Park, Sarawak (Malaysia) to reconstruct the spatio-temporal dynamics of sea surface temperature and oxygen isotopic composition of seawater from 1982 to 2016, based on paired oxygen isotope and Sr/Ca measurements. The results revealed rising sea surface temperatures of 0.26 ± 0.04 °C per decade since 1982. Reconstructed δ18Osw displayed positive excursion during major El Niño events of 1983, 1997/98 and 2015/16, indicating drought conditions with less river runoff, rainfall and higher ocean salinities. La Niñas were generally associated with lower δ18Osw. We observed a long-term shift from more saline conditions between 1982 and 1995 towards less saline conditions after 1995, which are in agreement with the regional freshening trend, punctuated by saline excursion during El Niños. The decadal shifts were found to be driven by the Pacific Decadal Oscillation (PDO). This study provides the first long-term data on El Niño Southern Oscillation (ENSO)-driven synchrony of climate impacts on both terrestrial and marine ecosystems in northern Borneo. Our results suggest that coral records from northern Borneo are invaluable archives to detect regional ENSO and PDO impacts, and their interaction with the Asian-Australian monsoon, on the hydrological balance in the southern South China Sea beyond the past three decades.

Simulation of early Eocene water isotopes using an Earth system model and its implication for past climate reconstruction

Our understanding of past climate conditions largely comes from paleoclimate proxies, such as oxygen isotope ratios (δ18Oc) in marine fossils. The marine δ18Oc signal primarily reflects a mixture of seawater temperature and oxygen isotopic composition of seawater (δ18Ow) at the time of calcification. Knowledge of δ18Ow is critical for the interpretation of marine δ18Oc records but remains poor for past hothouse climates. Here, we conduct water isotope-enabled simulations of the early Eocene using CO2 levels of 1×, 3×, 6×, and 9× the preindustrial value. We calculate model δ18Oc using simulated δ18Ow and ocean temperature, and make direct comparison with proxy records. Model δ18Oc matches the proxy values well for the early Eocene and Paleocene–Eocene Thermal Maximum with root-mean-squared errors approaching the standard error in individual records. Eocene δ18Ow in the model exhibits strong variation depending on states of the hydrological cycle and ocean circulation. Differences in the mean δ18Ow between regions of net evaporation and precipitation increase monotonically with the magnitude of the net atmospheric moisture transport that connects them; however, this relationship breaks at the regional scale due to ocean circulation changes. In particular, an increase in ocean ventilation brings more 18O-enriched deep water into the mixed layer, increasing sea-surface δ18Ow near the ventilation site and in certain remote regions through fast upper ocean currents. δ18Ow variations and the linkage to both hydrological cycle and ocean circulation bring challenges for an accurate interpretation of marine δ18Oc records. Our study illustrates the value of using water isotope-enabled simulations and model-data comparison for learning past climate changes.

The triple oxygen isotope composition of Precambrian chert

The temperature and chemistry of early seawater have both been inferred from the isotopic composition of Precambrian chert (SiO2), a precipitated mineral formed on or within marine sediments. The δ18O of chert shows a robust quasi-linear increase through time - a signal that has been interpreted in a number of conflicting ways. For example, changing δ18O has been hypothesized to reflect the product of cooling surface ocean temperatures, a signature of evolving seawater δ18O composition, or the product of later stage diagenesis (where measured δ18O reflects the composition of diagenetic fluids). We suggest this uncertainty can be resolved through the additional measurement and interpretation of the minor oxygen isotope 17O (noted as Δ'17O) in conjunction with δ18O. In this study, we present a suite of triple oxygen isotope data on stratigraphically constrained Precambrian chert (both peritidal chert nodules in carbonates and iron formation silica). These mineralogically well-defined data allow for the first stratigraphic tests of the fidelity of 17O in SiO2. We then apply a Monte Carlo resampling technique to test the features of the competing hypotheses noted above, here now including critical constraints from 17O. The most parsimonious interpretation of these data suggests that secondary alteration with higher-temperature, meteoric-derived groundwater has skewed an original geochemical signature. This treatment can allow for some change in the oxygen isotope composition of seawater itself, however this does not appear to be the most statistically defensible single solution - clearly, some combination of multiple mechanisms is always possible and even likely. What is definitively the case is an equitable, modern-like Archean surface ocean temperature.

Global Core Top Calibration of δ18O in Planktic Foraminifera to Sea Surface Temperature

AbstractThe oxygen isotopic composition of planktic foraminiferal calcite ( ) is one of the most prevalent proxies used in the paleoceanographic community. The relationship between , temperature, and seawater oxygen isotopic composition ( ) is firmly rooted in thermodynamics, and experimental constraints are commonly used for sea surface temperature (SST) reconstructions. However, in marine sedimentary applications, additional sources of uncertainty emerge, and these uncertainty constraints have not as of yet been included in global calibration models. Here, we compile a global data set of over 2,600 marine sediment core top samples for five planktic species: Globigerinoides ruber, Trilobatus sacculifer, Globigerina bulloides, Neogloboquadrina incompta, and Neogloboquadrina pachyderma. We developed a suite of Bayesian regression models to calibrate the relationship between and SST. Spanning SSTs from 0.0 to 29.5 °C, our annual model with species pooled together has a mean standard error of approximately 0.54‰. Accounting for seasonality and species‐specific differences improves model validation, reducing the mean standard error to 0.47‰. Example applications spanning the Late Quaternary show good agreement with independent alkenone‐based estimates. Our pooled calibration model may also be used for reconstruction in the deeper geological past, using modern planktic foraminifera as an analog for non‐extant species. Our core top‐based models provide a robust assessment of uncertainty in the paleothermometer that can be used in statistical assessments of interproxy and model‐proxy comparisons. The suite of models is publicly available as the Open Source software library bayfox, for Python, R, and MATLAB/Octave.

Intermediate water circulation changes in the Florida Straits from a 35 ka record of Mg/Li-derived temperature and Cd/Ca-derived seawater cadmium

We generated a high-resolution ocean record of bottom water temperature and seawater Cd from 546 m in the Florida Straits. The record extends to ∼35,000 yr before present, including all of Marine Isotope Stages (MIS) 2 and part of MIS 3. Previously, seawater cadmium (Cdw) has been used as a proxy for the strength of the Atlantic Meridional Overturning Circulation (AMOC) during the last deglaciation. We now consider the temperature and oxygen isotopic composition of seawater reconstructed from the same core in order to provide a fuller picture of water mass properties and circulation at this location. During the Last Glacial Maximum (LGM), Cdw levels were generally low, and a novel Mg/Li-derived temperature record reveals persistently cold temperatures (∼3.6°C, two to three degrees colder than during the Holocene or Marine Isotope Stage 3). During the Younger Dryas and early Heinrich Stadial 1, Cdw was low and temperatures were elevated compared with periods of inferred stronger AMOC. By contrast, there is no indication of AMOC variability over Heinrich Stadials 2 and 3, consistent with previous studies that conclude the AMOC is less sensitive to freshwater forcing during glaciations than during the last deglaciation. While there is some inconsistency between proxies, Cdw increases over some MIS3 Dansgaard–Oeschger interstadials, providing qualified support for a strengthening in AMOC. This is similar to our previous finding that Cdw tracks AMOC variability during the deglaciation. This study highlights the distinct nature of water masses and circulation during the LGM, relative to the stadial (weak AMOC) periods of the deglaciation and Late MIS 3.

Low and seasonally variable salinity in the Pennsylvanian equatorial Appalachian Basin

Use of the δ18O thermometer in deep time investigations is complicated by uncertainty in the oxygen isotopic composition of seawater and an increasing potential for diagenetic alteration with age. These concerns are particularly important when considering that δ18O values from Paleozoic marine carbonates tend to be low and increasingly depleted with age. Demonstrating the preservation of original chemistry and thus eliminating diagenesis as a factor is a primary hurdle in reconstructing climatic conditions in deep time. Here, we report stable isotope data from serially sampled, Middle Pennsylvanian mollusks from the tropical Appalachian Basin of Kentucky, USA. X-ray diffraction and scanning electron microscopy indicate an aragonite mineralogy with retention of primary microtextures, elemental chemistry is consistent with the shells of living mollusks, and δ18O data reveal regular cyclic variation over ontogeny, together suggesting that original shell carbonate is preserved and records environmental conditions over the life history of the animal. However, values are depleted, centering around −4.6‰, and intraannual variation is significant, spanning up to 2.2‰, calling for significant summertime runoff of fresh water to the basin in which the shell-producing organisms grew. Our data extend a documented trend toward more depleted carbonate values eastward across the midcontinent sea with increasing distance from Panthalassa, reflecting a salinity gradient driven largely by isotopically depleted and seasonal runoff from the Central Pangaean Mountains. Mollusk data support the primary nature of low δ18O values from brachiopod shells in the midcontinent, but they also highlight the problems with calculating paleotemperatures from δ18O values of carbonates precipitated in epicontinental seas by assuming a marine seawater composition. While carbonate clumped isotope paleothermometry is a potential solution to this seawater δ18O ‘problem’, we demonstrate that such data from these aragonitic samples are related to burial diagenesis and not the Pennsylvanian paleoenvironment. The bias toward samples from such settings in the Paleozoic, because continental margins are more likely to be deformed, suggests that postulated regional and global temperature histories should be treated with caution until epeiric oxygen isotope seawater values can be better constrained.

Winter iron supply processes fueling spring phytoplankton growth in a sub-polar marginal sea, the Sea of Okhotsk: Importance of sea ice and the East Sakhalin Current

To evaluate the winter iron (Fe) supply processes fueling spring phytoplankton growth in the southern Sea of Okhotsk, the distributions of the Fe concentrations in seawater were investigated using hydrographic observations in late November (in the absence of sea ice) and mid-February (in the presence of sea ice). The results obtained from the hydrographic observations indicated a temporal increase in the Fe concentration in the surface mixed layer from November to February, which coincided with a decrease in seawater salinity. Changes in the alkalinity and oxygen isotopic composition of seawater indicated that the mixed layer in February was affected by sea ice meltwater as well as the less saline East Sakhalin Current (ESC) water, with a significant Fe supply originating from these waters. The magnitude of the Fe supply from these waters is greater than the upward Fe supply from deep water. The sea ice meltwater enriches the Fe in the near-surface water of the mixed layer, whereas the ESC water enriches the Fe at the depth of the mixed layer by the entrainment of sedimentary Fe associated with deep mixing due to Ekman transport. Additionally, the Fe nutritional status of the phytoplankton in the surface water of the ice-covered area was investigated by a shipboard incubation experiment. The results showed that bio-available Fe existed in the surface water and that large (>10 μm) phytoplankton were not stressed by low Fe availability. The Fe supply processes related to sea ice melting and the ESC water inflow play important roles in sustaining the suitable Fe nutritional status of phytoplankton in the southern Sea of Okhotsk in early spring.

Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater

The evolution of the seawater oxygen isotopic composition (δ18O) through geological time remains controversial. Yet, the past δ18Oseawater is key to assess past seawater temperatures, providing insights into past climate change and life evolution. Here we provide a new and unprecedentedly precise δ18O value of −1.33 ± 0.98‰ for the Neoproterozoic bottom seawater supporting a constant oxygen isotope composition through time. We demonstrate that the Aït Ahmane ultramafic unit of the ca. 760 Ma Bou Azzer ophiolite (Morocco) host a fossil black smoker-type hydrothermal system. In this system we analyzed an untapped archive for the ocean oxygen isotopic composition consisting in pure magnetite veins directly precipitated from a Neoproterozoic seawater-derived fluid. Our results suggest that, while δ18Oseawater and submarine hydrothermal processes were likely similar to present day, Neoproterozoic oceans were 15–30 °C warmer on the eve of the Sturtian glaciation and the major life diversification that followed.