Carbonate Archives Research Articles

Paired X‐Ray Micro CT Scanning and Individual Foraminifera Isotopic Analysis Reveal (De)coupled Changes in Carbonate Preservation and Temperature

AbstractThe composition and preservation state of biogenic carbonate archives, such as foraminiferal tests, record ocean chemistry during the lifetime of the organism and post‐depositional changes in ambient conditions via carbonate compensation. Depending upon the specific paleoclimate proxy, post‐depositional processes, including dissolution, may alter original paleoenvironmental signals captured by the foraminifer's test composition. Accordingly, quantifying dissolution independent of geochemical measurements can improve proxy interpretation. Developing independent tools may also be useful for investigating whether changes in paleoclimatic conditions are associated with changes in seawater carbonate chemistry. Such approaches can be improved further if they are applied to individual foraminiferal tests, as specimen‐to‐specimen differences can record higher‐frequency environmental changes compared to conventional bulk‐scale analyses. Here, we combine individual foraminiferal carbon and oxygen isotopic analyses (IFA) with X‐ray MicroCT Scanning to generate paired analyses of test density (a proxy for the extent of post‐depositional dissolution) and isotopic composition. As a proof‐of‐concept application of this approach, we analyze Globigerina bulloides tests from both coretop and latest Miocene/earliest Pliocene‐aged sediment from Ocean Drilling Project (ODP) Site 1088 (Agulhas Ridge). Our measurements and mixing model calculations show that within‐population differences in carbon and oxygen isotopic ratios are largely independent of dissolution extent. By comparing population averages from coretop and downcore sediments, we find that lower oxygen isotopic ratios (likely driven by higher calcification temperatures) are associated with greater extents of dissolution at ODP Site 1088. We interpret this finding to reflect coupled changes in carbonate chemistry and climatic conditions over million‐year timescales.

Towards a better understanding of the geochemical proxy record of complex carbonate archives

Carbonate archives record a brief snapshot of the ambient Earth’s surface conditions at their deposition. However, the geologically reasonable extraction and interpretation of geochemical proxy data from ancient, diagenetically altered rock archives is fraught with problems. Three issues stand out: the dichotomy between petrographic and geochemical alteration; the lack of quantitative age constraints for specific diagenetic phases resulting in a poorly constrained admixture of local, basin-wide and over-regional (far-field) features; and an often insufficient understanding of the temperatures and compositions of diagenetic fluids. Here, the archive of Devonian marine limestones exposed to multiple far-field diagenetic events is used as an example to explore the above-listed issues. Methods applied include petrography, micro XRF, fluid inclusion data, clumped isotopes, δ13C and δ18O isotopes, 87Sr/86Sr ratios and quartz trace element data. Devonian limestones studied here were overprinted by two cross-cutting regional fault zones (T ≈ 230 °C) by multiple events between the Variscan Orogeny and the late Paleogene. The following processes are recorded: (i) protolith deposition and partial dolomitisation during rapid burial in the Middle/Late Devonian (T ≈ 180 °C); (ii) deep burial to ca 6.5 km and tectonic/hydrothermal overprint during the Variscan Orogeny in the Carboniferous (T ≈ 90–230 °C); (iii) rapid uplift to 1–2 km burial depth at the end of the Variscan Orogeny and hypogene karstification (T ≈ 50 to 100 °C) initiated by regional geology in the Permian/Triassic; (iv) tectonic/hydrothermal overprint during the opening of the Proto-Atlantic Ocean between the Early Jurassic and the Early Cretaceous (T ≈ 50 to 130 °C); (v) tectonic/hydrothermal overprint including renewed hypogene karstification and hydrothermal calcite cement precipitation (T ≈ 50 to 180 °C) during Alpine Orogeny between the Late Cretaceous and late Paleogene. Despite this complex series of diagenetic events, the protolith limestones largely preserved their respective Middle/Late Devonian dissolved inorganic carbon (DIC) and 87Sr/86Sr signatures. This study documents that geochemical proxy data, placed into their petrographic, paleotemperature, and local to over-regional context, significantly increases the ability to extract quantitative information from ancient carbonate rock archives. Research shown here has wider relevance for carbonate archive research in general.

Writ in water—Unwritten histories obtained from carbonate deposits in ancient water systems

AbstractCalcium carbonate deposits from ancient water systems such as aqueducts are a hidden archive for archaeology and environmental sciences. These deposits formed wherever carbonate‐rich water was in contact with a water‐containing structure and recorded water composition, temperature, biological content, the operation or nonoperation of a water system segment, flow discharge and velocity, the shape of disappeared segments of water structures, the number of years a water supply system was active, disruptions of the water supply and water management such as repairs, adaptations and cleaning. Indirectly, urban development, resilience, population‐ and socioeconomic dynamics can be studied through the stratigraphy of carbonate in water systems. Carbonate archives can also give insight into long‐term changes in paleoclimate and on environmental pollution, deforestation, extreme floods, droughts, earthquakes and volcanic eruptions. Archaeological and environmental investigations of carbonate deposits can provide data with up to daily resolution over decades to centuries. Although absolute dating of carbonate from water systems is still problematic, each study on the aqueduct of an ancient city, together with its carbonate deposits, provides its own microstory in Roman life.

Changing Sub‐Surface Chemistry Resulting From a 26‐Million‐Year Unconformity: Porewater Chemistry From IODP Site U1553 in the South Pacific

AbstractWhereas changes in pore water chemistry are known to impact carbonate sediment geochemistry, little is known about the impact of long unconformities on carbonate alteration. IODP Site 378‐U1553 on the southern Campbell Plateau, with a 26‐million‐year, erosional unconformity, provides a key location for examining the impact of long‐term unconformities on sub‐surface chemistry and carbonate archives. This study examined 76 interstitial water samples for sulfate sulfur and oxygen isotopes, as well as 28 bulk carbonate samples for carbonate associated sulfate (CAS) sulfur isotopes, to quantify the effects of this unconformity on the sub‐surface redox chemistry of the Site. The current state of the system suggests limited influence of redox processes on the CAS archive. Manganese reduction reaches 30 mbsf, with a decrease in manganese reduction between 20 and 30 mbsf. Below 30 mbsf, the system transitions to iron reduction to a depth of approximately 140 mbsf where sulfate reduction begins. Dissolved sulfate sulfur and oxygen isotope values suggest repeated oxygenation of sulfides. The CAS record from the Site deviates from previously published seawater values. However, the lack of a relationship between the dissolved sulfate and CAS records suggests most of the alteration of the CAS record likely occurred before the unconformity when the carbonate sediments were more reactive. This further supports the CAS record as a relatively robust archive, withstanding most post‐depositional mechanisms of carbonate alteration.

Multi-proxy assessment of brachiopod shell calcite as a potential archive of seawater temperature and oxygen isotope composition

Abstract. Most of our knowledge of past seawater temperature history is based on δ18O values of calcium carbonate fossil shells. However, the determination of past temperatures using this proxy requires the knowledge of past seawater δ18O values, which is generally poorly constrained. Other paleothermometers using carbonate archives, such as Mg/Ca ratios and clumped isotopes (Δ47), have been developed to allow for paleotemperatures to be estimated independently and to allow past ocean δ18O values to be calculated using various groups of calcifying organisms. Articulated brachiopod shells are some of the most commonly used archives in studies of past oceanic geochemistry and temperature. They are abundant in the fossil record since the Cambrian, and for decades, their low Mg–calcite mineralogy has been considered relatively resistant to diagenetic alteration. Here, we investigate the potential of brachiopod shells as recorders of seawater temperatures and seawater δ18O values using new brachiopod shell geochemical data by testing multiple well-established or suggested paleothermometers applied to carbonate archives. Modern articulated brachiopod shells covering a wide range of temperatures (−1.9 to 25.5 ∘C), depths (5 to 3431 m) and salinities (33.4 to 37.0 PSU) were analysed for their stable isotope compositions (δ13C, δ18O and Δ47) and their elemental ratios (Mg/Ca, Sr/Ca, Na/Ca and Li/Ca). Our data allowed us to propose a revised oxygen isotope fractionation equation between modern-brachiopod shell calcite and seawater: (1) T = - 5.0 ( ± 0.2 ) ( δ 18 O c - δ 18 O sw ) + 19.4 ( ± 0.4 ) , where δ18Oc is in ‰ VPDB, δ18Osw is in ‰ VSMOW, and T is in ∘C. Our results strongly support the use of clumped isotopes as an alternative temperature proxy but confirm significant offsets relative to the canonical relationship established for other biogenic and abiogenic calcium carbonate minerals. Brachiopod shell Mg/Ca ratios show no relationship with seawater temperatures, indicating that this ratio is a poor recorder of past changes in temperatures, an observation at variance with several previous studies. Despite significant correlations with brachiopod living temperature, brachiopod shell Sr/Ca, Na/Ca and Li/Ca values indicate the influence of environmental and biological factors unrelated to temperature, which undermines their potential as alternative temperature proxies. Kinetic effects (growth rates) could explain most of the deviation of brachiopod shell calcite from expected isotopic equilibrium with seawater and part of the distribution of Sr/Ca, Na/Ca and Li/Ca ratios.

Environmental Changes Recorded in Tufa from the Korana River, Croatia: Geochemical and Isotopic Approach

Between 1979 and 2003, a 35 cm thick layer of laminated tufa formed around a wooden pillar located at the beginning of the Korana River, which emerges from the Plitvice Lakes, Croatia. The laminated tufa structure allowed for the stratigraphic analysis of mineral, elemental and δ18O and δ13C isotope composition. Results are compared to other carbonate archives (lake sediments) from the Plitvice Lakes and to measured water physical–chemical data collected in the same time. An increase in water temperature and decrease in Mg/Ca ratio was observed during that period. Results showed that the tufa Mg/Ca ratio and δ18O could not be used as a temperature proxy, while an observed δ18O increase after 1990 was attributed to increase in evaporation rate. Furthermore, the constant increase in redox-sensitive elements (Mn) towards the external part of tufa may indicate periodic events of anoxia at the sediment/water interface. Local enrichment factors (LEFs) for potentially toxic elements were calculated using the background function and Al as the normalizing element. The LEFs showed an enrichment pattern of Cu and Pb in 1983 and 2001, which could indicate their anthropogenic origin and the sensitivity of the tufa to environmental changes.

Cadmium isotope systematics in sedimentary carbonate: Extending the utility of the cadmium isotope palaeo-productivity proxy

The cadmium (Cd) isotope compositions of seven Holocene (<11.7 ka) carbonate sediments, including the bulk carbonate, as well as the > 140 μm (zooplankton), <63 μm and < 20 μm (phytoplankton) fractions, and individual species of foraminifera isolated from the bulk carbonate were obtained for sub-samples processed using the Cd-cleaning and Mg/Ca cleaning methods. The respective Cd isotope systematics of each carbonate archive were then compared with the Cd isotope signature within the ocean using modern seawater data. We show that a constant Cd isotope fractionation factor of −0.25 ± 0.03 ‰ (2SE, n = 4) describes the partitioning of Cd from seawater into the carbonate phase (Δ114CdCdCO3-seawater), allowing for accurate reconstructions of the past seawater nutrient regime. The Cd isotope signature of the < 63 µm and < 20 µm (phytoplankton) fractions can display comparable values to those recorded in the modern surface ocean, and can be used to model the magnitude of Cd uptake and utilisation in the past ocean. Individual species of planktic and benthic foraminifera, which inhabit different depths of the water column, can provide a full surface to deep water column reconstruction of the Cd nutrient regime, while infaunal benthic foraminifera can be used to interrogate the Cd isotope systematics of sediment porewaters. Bulk sediment and the > 140 µm (zooplankton) fraction also retain important information relating to the overlying water column Cd isotope systematics and can serve as essential archives for palaeo-productivity reconstructions where the separation of individual components from the bulk sediment is not possible. This new Cd isotope palaeo-productivity toolkit can considerably aid reconstruction efforts and vastly improves the ability to quantify the role of phytoplankton in modulating carbon sequestration through transitions in Earth’s climate evolution.

238U, 235U and 234U in seawater and deep-sea corals: A high-precision reappraisal

Uranium isotope ratios are widely utilized in paleoceanography. The 238U/235U ratio (expressed as δ238U) is leveraged as a proxy for the areal extent of anoxic seafloor, and the 234U/238U ratio (expressed as δ234Usec) tracks riverine and estuarine inputs to the ocean, in addition to featuring prominently in U-series geochronology. Both of these ratios are thought to be recorded by biological carbonates precipitating from seawater, with corals serving as one of the most commonly-used archives of seawater U isotope ratios in the past. The utility of the U isotope proxy in biological carbonate archives relies not only on this faithful archiving of ambient seawater signatures, but also on the homogeneity of the seawater U isotope composition, which enables samples to be leveraged as proxy for the entire ocean.Here we revisit the foundational assumption of homogeneity of the marine U reservoir, and the capacity of deep-sea corals to record the U isotopic composition of ambient seawater. We begin by evaluating the analytical limits of precision and accuracy achievable for both δ238U and δ234Usec analysis by MC-ICP-MS. We then report data for 45 seawater and 26 deep-sea coral samples from multiple sites around the world. We find subtle δ238U and δ234Usec heterogeneity that correlates with U concentrations, which allows us to calculate new salinity-normalized global mean seawater values for δ238U (−0.379 ± 0.023 ‰) and δ234Usec (+145.55 ± 0.28 ‰). At each site, biological carbonates act as precise archives of the seawater δ238U value. The same is true for δ234Usec, with a few exceptions where samples appear to show vital effects that cause intra-sample 234U/238U re-partitioning. In sum, these observations support deep-sea corals as a robust archive of seawater U isotope ratios, but highlight the importance of utilizing multiple sample sites and replicate analyses to overcome coral vital effects (for δ234Usec) and subtle marine U isotopic heterogeneity.

Comparing isotopic estimates of paleoelevation from carbonates and volcanic glass from the Miocene-age Chucal Formation in northern Chile

Paleoaltimetry reconstructions of the Central Andes based on stable isotope data indicate variable timing and rates of uplift, reflecting the operation of multiple geodynamic mechanisms. There are a range of isotopic methods and proxies involved in these reconstructions, and it may be that biases inherent in some of these methods has led to misinterpretations. Here we explored the limitations of carbonate archives in reconstructing past elevations, including underestimation of paleoelevations by oxygen-isotope evaporative enrichment and carbonate formation in the subsurface. To conduct our test, we obtained stable isotope values of carbonates from the Western Cordillera in northern Chile at ~18.5–19.5°S, where previously published paleoaltimetry results from the same sections, based on the hydrogen stable isotope composition of volcanic glass, indicate that this region of the Andes has remained at high elevations (>4.4 km) since at least ~23 Ma. We present new oxygen and carbon isotopic, and clumped isotopic results, petrographic and sedimentologic observations, as well as major element abundance (ICP-MS and microprobe analyses) from lacustrine carbonates (marlstones) and cements in sandstones of the early Miocene Chucal Formation in the central Andes of northern Chile. The clumped isotope paleotemperature estimates range from 26 to 45 °C in lacustrine carbonates and 27 to 54 °C in cements. These results indicate that carbonates from this section do not reflect surface temperatures, suggesting clumped isotope resetting in a rock-buffered setting for marlstones and carbonate formation at depth in the presence of groundwater for cements in sandstones. In contrast, the oxygen and carbon isotopic values from carbonates appear to be primary and combined with Mg/Ca ratios and Na concentrations document the transition from lake deposition into a dominantly fluvial environment within the Chucal Formation, also indicating variable evaporative conditions during deposition. The paleoaltimetry estimates in carbonates significantly underestimate the results determined in volcanic glass, with paleoelevation results ranging between 1.4 km and 2.6 km in marlstones and from 2.6 and 3.6 km in cements.. The results presented in this study highlight the importance of using multiple proxies for paleoaltimetry reconstructions, especially in arid land settings where meteoric waters have been modified by evaporation.

ShellChron 0.4.0: a new tool for constructing chronologies in accretionary carbonate archives from stable oxygen isotope profiles

Abstract. This work presents ShellChron, a new model for generating accurate internal age models for high-resolution paleoclimate archives, such as corals, mollusk shells, and speleothems. Reliable sub-annual age models form the backbone of high-resolution paleoclimate studies. In the absence of independent sub-annual growth markers in many of these archives, the most reliable method for determining the age of samples is through age modeling based on stable oxygen isotope or other seasonally controlled proxy records. ShellChron expands on previous solutions to the age model problem by fitting a combination of a growth rate and temperature sinusoid to model seasonal variability in the proxy record in a sliding window approach. This new approach creates smoother, more precise age–distance relationships for multi-annual proxy records with the added benefit of allowing assessment of the uncertainty in the modeled age. The modular script of ShellChron allows the model to be tailored to specific archives, without being limited to oxygen isotope proxy records or carbonate archives, with high flexibility in assigning the relationship between the input proxy and the seasonal cycle. The performance of ShellChron in terms of accuracy and computation time is tested on a set of virtual seasonality records and real coral, mollusk, and speleothem archives. The result shows that several key improvements in comparison to previous age model routines enhance the accuracy of ShellChron on multi-annual records while limiting its processing time. The current full working version of ShellChron enables the user to model the age of 10-year-long high-resolution (16 samples yr−1) carbonate records with monthly accuracy within 1 h of computation time on a personal computer. The model is freely accessible on the CRAN database and GitHub. Members of the community are invited to contribute by adapting the model code to suit their research topics and encouraged to cite the original work of Judd et al. (2018) alongside this work when using ShellChron in future studies.

Last glacial temperature reconstructions using coupled isotopic analyses of fossil snails and stalagmites from archaeological caves in Okinawa, Japan

We applied a new geoarchaeological method with two carbonate archives, which are fossil snails from Sakitari Cave and stalagmites from Gyokusen Cave, on Okinawa Island, Japan, to reconstruct surface air temperature changes over the northwestern Pacific since the last glacial period. Oxygen isotope ratios (δ18O) of modern and fossil freshwater snail shells were determined to infer seasonal temperature variations. The observational and analytical data confirm that δ18O values of fluid inclusion waters in the stalagmite can be regarded as those of spring waters at the sites where snails lived. Our results indicate that the annual mean, summer, and winter air temperatures were lower by 6–7 °C at ca. 23 thousand years ago (ka) and 4–5 °C at ca. 16–13 ka than those of the present day. Our reconstruction implies that surface air cooling was possibly two times greater than that of seawater around the Ryukyu Islands during the Last Glacial Maximum, which potentially enhanced the development of the East Asian summer monsoon during the last deglaciation. Considering the potential uncertainties in the temperature estimations, the climatic interpretations of this study are not necessarily definitive due to the limited number of samples. Nevertheless, our new geoarchaeological approach using coupled δ18O determinations of fossil snails and stalagmite fluid inclusion waters will be useful for reconstructing snapshots of seasonally resolved time series of air temperatures during the Quaternary.

Constraints on the preservation of proxy data in carbonate archives – lessons from a marine limestone to marble transect, Latemar, Italy

AbstractThis work evaluates an exceptionally complex natural laboratory, the Middle Triassic Latemar isolated platform in the northern Italian Dolomite Mountains and explores spatial and temporal gradients in processes and products related to contact metamorphism, dolomitization and dedolomitization of marine limestones. The relation between petrographic change and re‐equilibration of geochemical proxy data is evaluated from the perspective of carbonate‐archive research. Hydrothermal dolomitization of the limestone units is triggered by dykes and associated hydrothermal fluids radiating from the nearby Predazzo Intrusion. Detailed petrography, fluid inclusion analysis, δ13C and δ18O data and 87Sr/86Sr isotope ratios shed light on the extreme textural and geochemical complexity. Metamorphic and diagenetic patterns include: (i) peak‐metamorphic and retrograde‐metamorphic phases including three dolomite marbles, two dedolomite marbles, brucite, magnesium silicates and late‐stage meteoric/vadose cement at the contact aureole; (ii) four spatially defined episodes of dolomitization, authigenic quartz, low magnesium calcite and late‐stage meteoric cement at the Latemar isolated platform; and (iii) kilometre‐scale gradients in δ13C values from the contact aureole towards the platform interior. Results shown here are relevant for two reasons: first, the spatial analysis of alteration products ranging from high‐grade metamorphic overprint of marbles at temperatures of 700°C in the contact aureole to moderately altered limestones in the platform interior at temperatures &lt;100°C, allows the observation of processes that commonly occur along vertical (prograde) gradients from shallow burial to metamorphism at depths &gt;20 km. Second, under rock‐buffered conditions, and irrespective of metamorphic to diagenetic fluid−rock interactions, both marbles, and low‐temperature hydrothermal dolomites have conservative marine δ13C and δ18O values. The fact that metamorphism and hydrothermal dolomitization of precursor limestones and early diagenetic dolostones did not per se reset environmental proxy data is of interest for those concerned with carbonate archive research in Earth’s deep time.

Supplement to: "ShellChron: A new tool for constructing chronologies in accretionary carbonate archives from stable oxygen isotope profiles" by Niels J. de Winter

Supplement to: "ShellChron: A new tool for constructing chronologies in accretionary carbonate archives from stable oxygen isotope profiles" by Niels J. de Winter

Supplement to: "ShellChron: A new tool for constructing chronologies in accretionary carbonate archives from stable oxygen isotope profiles" by Niels J. de Winter

Supplement to: "ShellChron: A new tool for constructing chronologies in accretionary carbonate archives from stable oxygen isotope profiles" by Niels J. de Winter

Supplement to: "ShellChron: A new tool for constructing chronologies in accretionary carbonate archives from stable oxygen isotope profiles" by Niels J. de Winter

Supplement to: "ShellChron: A new tool for constructing chronologies in accretionary carbonate archives from stable oxygen isotope profiles" by Niels J. de Winter

Rise of the Colorado Plateau: A Synthesis of Paleoelevation Constraints From the Region and a Path Forward Using Temperature-Based Elevation Proxies

The Colorado Plateau’s complex landscape has motivated over a century of debate, key to which is understanding the timing and processes of surface uplift of the greater Colorado Plateau region, and its interactions with erosion, drainage reorganization, and landscape evolution. Here, we evaluate what is known about the surface uplift history from prior paleoelevation estimates from the region by synthesizing and evaluating estimates 1) in context inferred from geologic, geomorphic, and thermochronologic constraints, and 2) in light of recent isotopic and paleobotanical proxy method advancements. Altogether, existing data and estimates suggest that half-modern surface elevations were attained by the end of the Laramide orogeny (∼40 Ma), and near-modern surface elevations by the mid-Miocene (∼16 Ma). However, our analysis of paleoelevation proxy methods highlights the need to improve proxy estimates from carbonate and floral archives including the ∼6–16 Ma Bidahochi and ∼34 Ma Florissant Formations and explore understudied (with respect to paleoelevation) Laramide basin deposits to fill knowledge gaps. We argue that there are opportunities to leverage recent advancements in temperature-based paleoaltimetry to refine the surface uplift history; for instance, via systematic comparison of clumped isotope and paleobotanical thermometry methods applied to lacustrine carbonates that span the region in both space and time, and by use of paleoclimate model mediated lapse rates in paleoelevation reconstruction.

Supplement to: "ShellChron: A new tool for constructing chronologies in accretionary carbonate archives from stable oxygen isotope profiles" by Niels J. de Winter

Supplement to: "ShellChron: A new tool for constructing chronologies in accretionary carbonate archives from stable oxygen isotope profiles" by Niels J. de Winter

The aqueduct of Gerasa – Intra-annual palaeoenvironmental data from Roman Jordan using carbonate deposits

Calcium carbonate (CaCO3) deposits from Roman aqueducts are an innovative archive to obtain local high-resolution palaeoenvironmental and archaeological data in interdisciplinary studies. Deposits from one of the aqueducts of the Roman city of Gerasa provide a record of 59 years during the 1st to 3rd centuries CE, divided into three sequences separated by plaster layers. Annual carbonate layers show an alternation of sparite, formed in winter, and micrite, formed in summer. Brown bands at the base of many sparite layers probably correspond to large rainstorms in early winter. A fine lamination present in the brown bands may be diurnal in origin. Stable isotope and trace element data confirm annual layering, indicate strongly variable flow rate in the aqueduct and show truncations that may have been associated with drying up of the channel in some years. The trace element pattern is typical of a relatively small aquifer with a rapid response to precipitation. The trace element composition changes abruptly from the first to the second carbonate sequence, suggesting that a spring was added to increase the flow rate. Deformation twins in calcite crystals at the top of the second sequence may be due to earthquake damage after 48 years of use. The presence of abundant clay in the carbonate sequence, especially in the third sequence, suggests seismic damage to the channel. The channel was usually replastered after damage. The aqueduct went out of use sometime after the mid-2nd to mid-3rd century CE. The carbonate archive stores key information on groundwater quantity and composition and indirectly on air temperature, rainfall, extreme environmental events and land use at sub-annual resolution.

Chemical oceanographic influences on sediment accumulations of a carbonate ramp: Holocene Yucatan Shelf, Mexico

AbstractGlobal controls on the oceanographic influences on the nature of carbonate factories are broadly understood. The details of the influences of changes in temperature and nutrients across individual carbonate shelves are less well constrained, however. This study explores spatial and temporal variations in chemical oceanography along and across the Yucatan Shelf, a modern carbonate ramp, and how these factors relate to variable bottom character, sediment and sediment geochemistry. In‐situ sensors and remote‐sensing data indicate the sporadic presence of cool, upwelled water with low dissolved oxygen and elevated Chlorophyll‐a. This current‐driven, westward flow of upwelled water is most evident in a zone just offshore of the northern peninsular shoreline, but its influence wanes ca 75 km offshore and as the shore turns southward. The impacts of this water mass include a transitional photozoan–heterozoan assemblage with biosiliceous components, relict grains and common thin Holocene sediment accumulations nearshore; further offshore are coralgal reefs and expansive sand plains. Geochemical proxies of bulk sediment, including high δ18O and elevated HREE/LREE (heavy rare‐earth element/light rare‐earth element) ratios near, and downcurrent of, the upwelling source, are interpreted to represent the signal of nearshore, westward movement of the cool and nutrient‐rich, upwelled water. Collectively, these data emphasize how local processes such as upwelling and longshore transport can variably influence carbonate sediment accumulations and their geochemical signatures, both along and across individual shelves. These data and insights provide an analogue for the influences of spatial variability of water masses in the geological record, and for accurate interpretation of stratigraphic changes of sedimentary and geochemical proxy data in carbonate archives.

Dual clumped isotope thermometry resolves kinetic biases in carbonate formation temperatures

Surface temperature is a fundamental parameter of Earth’s climate. Its evolution through time is commonly reconstructed using the oxygen isotope and the clumped isotope compositions of carbonate archives. However, reaction kinetics involved in the precipitation of carbonates can introduce inaccuracies in the derived temperatures. Here, we show that dual clumped isotope analyses, i.e., simultaneous ∆47 and ∆48 measurements on the single carbonate phase, can identify the origin and quantify the extent of these kinetic biases. Our results verify theoretical predictions and evidence that the isotopic disequilibrium commonly observed in speleothems and scleractinian coral skeletons is inherited from the dissolved inorganic carbon pool of their parent solutions. Further, we show that dual clumped isotope thermometry can achieve reliable palaeotemperature reconstructions, devoid of kinetic bias. Analysis of a belemnite rostrum implies that it precipitated near isotopic equilibrium and confirms the warmer-than-present temperatures during the Early Cretaceous at southern high latitudes.