ABSTRACT Carbonate concretions collected from the Dominican Republic present a valuable opportunity to evaluate the internal isotopic variations within concretions that have never been exposed to deep burial or structural deformation. Here, three concretions from the Neogene (Late Miocene–Early Pliocene) Cibao Basin are investigated, utilizing a multi‐isotope (δ 13 C, δ 18 O, δ 34 S CAS and ∆ 47 values) high‐resolution approach, to constrain the microenvironmental conditions associated with multiple stages of concretion growth. Isotopic variability and potential disequilibrium effects, which can influence geological interpretations utilizing concretions, are also considered. The petrographic characteristics and geochemical profiles indicate internal differences relating to concretion growth mechanisms and environmental changes, driven by sea‐level fluctuations. The δ 34 S values of carbonate‐associated sulphate indicate a closed system environment; however, the overall values are influenced by sulphide oxidation within the sediments, resulting in a complex signal. The ∆ 47 ‐derived temperatures of the concretions range between 29 to 55°C, indicating significantly warmer temperatures than are measured from the host sediments, which average 24°C. This indicates that carbonate concretion ∆ 47 values are in disequilibrium with their environments of formation, likely related to ion diffusion in the pore fluids or isotopic fractionation associated with microbial processes. Here geochemical variations within concretions are utilized to assess the environmental conditions and microbial interactions after sediment deposition. However, for future studies, caution should be taken when using concretions for making environmental assessments as the signals can be influenced by a multitude of processes, even prior to diagenetic alteration.

The geochemical records of shallow-water carbonate sediments can be influenced by both meteoric and marine diagenesis, where the contribution from each can be difficult to constrain using traditional δ18O and δ13C values. Here sediment cores from Enewetak Atoll have been utilized as a case study, in which clumped isotope-derived temperatures and calculated δ18Ofluid values are combined with the δ34S values of the carbonate-associated sulfate, to differentiate the types of diagenetic alteration which have influenced the geochemical record, thus providing further insights into the extent and timing of carbonate alteration. While sediments at Enewetak Atoll have undergone repeated periods of subaerial exposure, calculated δ18Ofluid values indicate that the amount of meteoric alteration is reliant on the availability of meteoric fluids, as the presence of exposure surfaces alone does not imply that extensive diagenetic alteration has occurred. Additionally, in the deepest zone of the Enewetak cores, it could be determined that the sediments were initially exposed to meteoric fluids, resulting in partial alteration, and later affected by burial and circulation of deep ocean water, resulting in recrystallization within the section, uniformly overprinting the temperature and δ18Ofluid values. The results of this study indicate that by utilizing systematics such as δ34SCAS, ∆47, and δ18Ofluid values, in addition to the more traditional proxies, a detailed understanding of the diagenetic processes which have altered shallow water sediments can be better constrained, allowing for more accurate interpretations of ancient sedimentary records.

Global correlation of negative excursions in the δ13C values of shallow water carbonates have been used to interpret and identify major environmental changes through geologic time, such as the glaciations of the Neoproterozoic. As a result of their global reproducibility and similar geometry, these signals have predominantly been interpreted as being representative of the original δ13C values of dissolved inorganic carbon in the oceans, which in turn reflect major changes in the global carbon cycle. In this paper δ13C values from cores drilled in Enewetak Atoll (Pacific) and the Great Bahama Bank (Atlantic), are shown as examples of how similarly correlating variations in global δ13C values, can arise from diagenetic processes. It is further shown that while the profiles of the changes in δ13C values may appear synchronous, as a result of varying rates of deposition and subsidence, that the actual timing of the two changes can be quite different. By comparison of the geometry and global correlation of δ13C values measured in this study, it is proposed that the δ13C values of ancient sequences, may have resulted from non-synchronous diagenetic processes, rather than true changes in the global carbon system. This emphasizes the need for precise chronological control when interpreting sediment records, which is not present during many older geological time periods, in order to constrain the origin and correlation of carbon isotope variations.

Rationale Information on the temperature of formation or alteration of carbonate minerals can be obtained by measuring the abundance of the isotopologues 47 and 48 (Δ 47 and Δ 48 values) of CO 2 released during acid dissolution. The combination of these two proxies can potentially provide a greater insight into the temperature of formation, particularly if the carbonate minerals form by non‐equilibrium processes. Methods We have precipitated calcium carbonates at seven temperatures between 5 and 65°C and measured their δ 48 values using a Thermo‐253 plus isotope ratio mass spectrometer. The values were transformed to Δ 48 values in the conventional manner and then converted to the carbon dioxide equilibrium scale. Results Using the Δ 48 values, we have established an empirical calibration between temperature and Δ 48 values: urn:x-wiley:09514198:media:rcm9147:rcm9147-math-0001 Conclusions The calibration line produced allows the determination of the temperature of natural carbonates using the Δ 48 values and agrees with the measurements of the Δ 47 and Δ 48 values of some carbonates assumed to have formed under equilibrium conditions.

Marine carbonate sediment cores have been extensively used for the reconstruction of paleo-environmental conditions, with isotopic proxies such as δ 13 C values being used to interpret significant global changes in the carbon cycle throughout earth’s history. Shallow water carbonates are often utilized for these reconstructions due to their accessibility and long-term preservation in the geologic record. Many previous studies have identified major δ 13 C perturbations within shallow-water carbonate systems, and argued that because these signals are global they reflect true changes in the ocean carbon cycle. However, diagenetic events such as sea-level change, can result in alteration of shallow water carbonates across all ocean basins, a signal which could be misinterpreted as true global carbon shifts. Evidence of sea-level related meteoric diagenesis, resulting in a correlating carbon isotope excursion across shallow water carbonates from the Bahamas has been shown, however, correlating this record globally proved difficult as the Pacific Ocean core was not fully sampled resulting in a short record [1]. This study thus investigates shallow water carbonate cores from the Atlantic (Great Bahama Bank) and Pacific (Enewetak Atoll) Ocean basins, which have been unequivocally influenced by Pleistocene sea-level fluctuations, to establish the role of sea-level in producing C and O isotopic signals which correlate on a global scale. This comparison is of further interest because while the Great Bahama Bank did not ‘keep up’ with sea-level changes, the atolls in the Pacific always ‘kept up’ with sea-level. This allowed the Pacific atolls not only to produce carbonate buildups higher than present-day sea-level, but also for these buildups to be weathered away as sea-level fell. Furthermore, due to their close proximity to the surface, the Pacific Atolls were influenced by many more sea-level changes than for example, the Bahamas

The Gulf of Mexico (GoM) is home to the world's largest remaining wild oyster fisheries, but baseline surveys needed to assess habitat condition are recent and may represent an already-shifted reference state. Here, we use prehistoric oysters from archaeological middens to show that oyster size, an indicator of habitat function and population resilience, declined prior to the earliest assessments of reef condition in an area of the GoM previously considered pristine. Stable isotope sclerochronlogy reveals extirpation of colossal oysters occurred through truncated life history and slowed growth. More broadly, our study suggests that management strategies affected by shifting baselines may overestimate resilience and perpetuate practices that risk irreversible decline.

Inorganic aragonite occurs in a wide spectrum of depositional environments and its precipitation is controlled by complex physio‐chemical factors. This study investigates diagenetic conditions that led to aragonite cement precipitation in Cenozoic glaciomarine deposits of McMurdo Sound, Antarctica. A total of 42 sandstones that host intergranular cement were collected from the CIROS‐1 core, located proximal to the terminus of Ferrar Glacier. Standard petrography, Raman spectroscopy and electron microprobe analysis reveal a prominent aragonite cement phase that occurs as a pore‐filling blocky fabric throughout the core. Oxygen isotope compositions (δ18O = −30·0 to −8·6‰ Vienna Pee‐Dee Belemnite) and clumped isotope temperatures (TΔ47 = 13·1 to 31·5°C) determined from the aragonite cements provide precise constraints on isotopic compositions (δ18Ow) of the parent fluid, which mostly range from −10·8 to −7·2‰ Vienna Standard Mean Ocean Water. The fluid δ18Ow values are consistent with those of pore water, previously identified as cryogenic brine in the nearby AND‐2A core. Petrographic and geochemical data suggest that aragonite cement in the CIROS‐1 core precipitated from a similar brine. The brine likely formed and infiltrated sediments in flooded glacial valleys along the western margin of McMurdo Sound during the middle Miocene Climatic Transition, and subsequently flowed basinward in the subsurface. Consequently, the brine forms as a longstanding subsurface fluid that has saturated Cenozoic sediments below southern McMurdo Sound since at least the middle Miocene. Aragonite cementation in the CIROS‐1 core is interpreted to reflect its proximal position to sites of brine formation and greater likelihood of experiencing brines with sustained high carbonate saturation states and Mg/Ca ratios. This unusual occurrence expands the range of known natural occurrences of aragonite cement. Given the potential for cryogenic brine formation in glaciomarine settings, blocky aragonite, as the end member of the spectrum of aragonite cement morphology, may be more widespread in glaciomarine sediments than currently thought.

This study reports the first radon concentration measurements within three frequently visited caves in Florida: one touristic (Florida Caverns), one private (Ocala Caverns), and one private, but publicly accessible (Jennings Cave). To measure the radon concentration, 18 CR-39 solid-state alpha track detectors were placed along the main passages of these caves for a period of 2 months (between December 2016 and February 2017). The results show that the radon concentration throughout all caves greatly exceeds the recommended safety action level. The highest concentrations of 2737 and 2958 Bq m−3 were recorded in Ocala and Jennings caves, respectively; whereas in Florida Caverns, the concentration reached a value as high as 1050 Bq m−3. To aid in ventilation, allowing the built-up gas to disperse, it is suggested that at Florida Caverns, the entry doors to be periodically opened for several hours. In locations with high concentrations where additional ventilation is not possible, such as Ocala and Jennings, it is recommended that the exposure time to be limited. Although radon values measured in the surveyed caves are high, the occasional cave visits are generally safe as the overall exposure time is minimal. However, cave guides and workers may have an increased risk as they spend many hours a day during which they are exposed to these high radon concentrations.