A 240-Year Stable Oxygen and Carbon Isotopic Record in a Coral from South Florida: Implications for the Prediction of Precipitation in Southern Florida

This study reports on the δ18O and δ13C composition of the skeleton from a 240-year-old specimen of Montastraea faveolata growing in Biscayne National Park, South Florida. Annual variations in the δ18O of the skeleton deposited during the summer months show a bimodal correlation with summer rainfall. During wetter years, the δ18O of the coral skeleton and the amount of precipitation during the summer months are inversely correlated (r = -0. 7) reflecting dilution of the seawater by meteoric water lower in δ18O. During years in which summer rainfall is less than normal, increases in precipitation are positively correlated with skeletal δ18O (r = +0.6) reflecting the input of freshwater from the Everglades higher in δ18O. Based on this correlation the δ18O record of the coral skeleton suggests that the 19th and 18th centuries have been relatively dry compared to the 20th century. Carbon isotopic compositions of the skeleton are positively correlated with δ18O with the minimum in δ13C occurring several months after the minimum in δ18O. Since the mid 1930s there has been a decrease in δ13C of the skeleton. Explanations for this trend may be (1) it reflects the increased input of carbon derived from the destruction of terrestrial ecosystems, (2) its part of a long-term decrease in δ13C associated with increased addition of fossil fuel-derived CO2

Decade-Scale Trend in Sea Water Salinity Revealed Through δ18O Analysis of Montastraea annularis Annual Growth Bands

Stable oxygen isotope ratios (δ18O) of coral skeletons are influenced by ambient water temperature and by the oxygen isotope ratio in the surrounding sea water, which, in turn, is linked to evaporation (salinity) and precipitation. To investigate this relationship more thoroughly, we collected hourly temperature data from the Hen and Chickens Reef in the Florida Keys between 1975 and 1988 and compared them to the δ18O of Montastraea annularis skeleton that grew during the same interval. To ensure that we obtained the correct oxygen isotopic range in the skeleton we typically sampled the coral at a resolution of 20–30 samples in 1 year; in 1 year we sampled the coral at a resolution of 70 samples·year−1. Despite our high-resolution sampling, we were unable to obtain the full temperature-induced δ18O range in the skeleton. Our data suggest that, during the summer, evaporation causes isotopic enrichment in the water, partially masking the temperature-induced signal. Our data also show that oxygen isotopic composition of seawater at the reef has increased since 1981. This increase indicates that salinity has increased slightly during the past decade, perhaps as a result of increased evaporation in waters of Florida Bay and the Keys. This phenomenon is probably not caused by a decrease in the outflow of freshwater into Florida Bay from the Everglades but may be related to the measured deficit in precipitation that has occurred over the past decade

The Origin of Variations in the Isotopic Record of Scleractinian Corals: II. Carbon

This study examines the relationship between the δ13C of the skeleton of a zooxanthellate coral (Montastraea annularis) growing on the Florida Reef Tract and environmental variables (insolation and temperature), physiological variables (growth rate, respiration, calcification, and photosynthesis). Colonies of this species were grown in the field for a 2.5 year study period, during which the rates of photosynthesis, respiration, and calcification were measured on fifteen separate occasions, spaced approximately equally throughout the study period. The corals were stained with alizarin-red S within seven days after each set of physiological measurements. At the end of the period the corals were sacrificed and their skeletal extension, density, and skeletal δ13C determined. Despite substantial high-frequency variations, a strong seasonal cycle was evident in the skeletal δ13C records of all the corals throughout the experimental period. The skeletal δ13C and δ18O values varied approximately in phase, and showed a weak, but statistically significant positive relationship with each other. The δ13C of the coral skeletons, when corrected for changes in the δ13C of dissolved inorganic carbon (DIC), exhibited an inverse correlation with P/R, a finding opposite to what was expected based on current models of isotopic fractionation in coral skeletons. Although such findings tend to support the model of Erez (1978) that increases in photosynthesis act to isotopically deplete the δ13C of the coral skeleton, we note that the inverse association between δ13C and P/R arises because of a slight positive association between δ13C and respiration. We therefore believe that the association may be a result of seasonal variation in some parameters of the system which was not constrained in our study. Alternatives include (1) variations in the δ13C of the DIC which are translated into the δ13C of the food chain, (2) changes from heterotrophy to autotrophy, and (3) changes in the partitioning of δ13C between the zooxanthellae and the coral tissue. Based on previous studies which we have carried out we believe that changes in the skeletal δ13C are not related to sexual reproduction or growth rate. Contrary to previous work we were unable to measure any significant differences in the skeletal δ13C between the fast growing tops of the coral and the slower growing sides

The 13C Suess Effect in Scleractinian Corals Mirror Changes in the Anthropogenic CO2 Inventory of the Surface Oceans

New δ13C data are presented from 10 coral skeletons collected from Florida and elsewhere in the Caribbean (Dominica, Dominican Republic, Puerto Rico, and Belize). These corals range from 96 to 200 years in age and were collected between 1976 and 2002. The change in the δ13C of the skeletons from these corals between 1900 and 1990 has been compared with 27 other published coral records from the Atlantic, Pacific, and Indian Oceans. The new data presented here make possible, for the first time, a global comparison of rates of change in the δ13C value of coral skeletons. Of these records, 64% show a statistically significant (p \u3c 0.05) decrease in δ13C towards the modern day (23 out of 37). This decrease is attributable to the addition of anthropogenically derived CO2 (13C Suess effect) to the atmosphere. Between 1900 and 1990, the average rate of change of the δ13C in all the coral skeletons living under open oceanic conditions is approximately −0.01‰ yr−1. In the Atlantic Ocean the magnitude of the decrease since 1960,−0.019 yr−1 ±0.015‰, is essentially the same as the decrease in the δ13C of atmospheric CO2 and the δ13C of the oceanic dissolved inorganic carbon (−0.023 to −0.029‰ yr−1), while in the Pacific and Indian Oceans the rate is more variable and significantly reduced (−0.007‰ yr−1 ±0.013). These data strongly support the notion that (i) the δ13C of the atmosphere controls ambient δ13C of the dissolved inorganic carbon which in turn is reflected in the coral skeletons, (ii) the rate of decline in the coral skeletons is higher in oceans with a greater anthropogenic CO2inventory in the surface oceans, (iii) the rate of δ13C decline is accelerating. Superimposed on these secular variations are controls on theδ13C in the skeleton governed by growth rate, insolation, and local water masses

Utilization of Freshwater and Ocean Water by Coastal Plants of Southern Florida

The coastal vegetation of southern Florida is undergoing dramatic changes due to the instability of the ocean water-freshwater boundary. These vegetation changes will be determined by the response of each particular species to saline ocean water, particularly whether it can use ocean water or not. In this study, isotopic data were used to determine the relative usage of freshwater or ocean water by plants in the Florida keys. The results indicate that, with some exceptions, plants toward the interior of the keys were using freshwater while those toward the edge were using ocean water. A plot of the hydrogen and oxygen isotopic composition of the plant water yielded a mixing line between typical freshwater values and those of ocean water. In general, the isotopic ratios of stem water for species found in hardwood hammocks were confined to the freshwater end of the line, followed by values of stem water from mangrove margin species. found in mangroves, however, had water with extremely variable isotopic ratios, ranging from values typical of ocean water to values typical of freshwater. This variability is consistent with the hypothesis that mangroves are fully capable of growing in freshwater, but are limited to saline habitats because of competitive exclusion by fast-growing glycophilic plants

Holocene Paleohydrology of Little Salt Spring, Florida, Based on Ostracod Assemblages and Stable Isotopes

Ostracod assemblages and the δ18O and δ13C records of Cytheridella ilosvayi provide a ∼12,000 yr paleohydrological reconstruction of Little Salt Spring, a sinkhole lake and underwater archaeological preserve in west central Florida. The ostracod record documents relative changes in water temperature and hydrologic characteristics of the lacustrine system that are the result of warming air temperatures, changes in the relative contributions of input waters (shallow vs. deep groundwater), rainfall patterns and relative sea level rise during the Holocene. The data indicate that LSS was initially supplied by relatively low amounts of direct rainfall and surface runoff as the spring\u27s location was closer to the recharge area and far from the freshwater/saltwater interface. At about 11,000 yr BP, LSS became groundwater supported, initially by a shallow freshwater aquifer with low δ18O composition until ∼5700 yr BP, and after that time by 18O-enriched and increasingly mineralized groundwater originating from a deeper, carbonate aquifer as the regional water table rose bringing the saltwater interface closer to the location of LSS which at this time was becoming a groundwater discharge area. The data also shows that the most abrupt and pronounced hydrologic changes occurred during the Late Holocene with an interval of low δ18O values (∼− 2.86‰) between 2700 and 2000 yr BP, followed by a stepped increase in δ18O composition with maximum values (∼− 1.46‰) between 1100 and 900 yr BP. Over these periods to the present time the ostracod assemblage is characterized by brackish species (Cyprideis spp. and Limnocythere floridensis). It is inferred that the interval of decreased isotopic values indicates mixing with saline water during dry climatic conditions, while the interval of more enriched isotopic values suggest saltwater intrusion during a period of, presumably, a sea level highstand. Although some discrepancies exist between other records from the northern Caribbean region and the one from LSS with regards to the exact timing for the Late Holocene drying and subsequent increase in rainfall conditions, the differences between the records may be explained by greater continental influences on ocean–atmosphere interactions on the Florida peninsula, a highly complex groundwater mixing balance in LSS, and by a better constrained age-model in the other records

The Origin of Variations in the Isotopic Record of Scleractinian Corals: I. Oxygen

Previous investigations of the δ18O of the skeletons of Florida specimens of the reef coral Montastraea annularis have failed to produce the full temperature range suggested by calibration studies of other corals. Explanations for this phenomenon include different relationships between temperature and the δ18O of skeletons of Floridian corals, changing δ18O of the water, physiological variables (“vital effects”), and an insufficient number of samples taken per year with consequent superposition of calcium carbonate precipitated at different times within an individual sample. In this study, we investigate all of these hypotheses, by measuring the δ18O of corals grown in the field which were periodically stained with alizarin-red S and where the δ18O of the water was measured and the temperature continuously recorded. We compare the effect of sampling the coral skeletons at different resolutions and the effect of sampling within different skeletal elements. Our study shows that discrete, high-resolution sampling of coral exotheca (fifty samples a year) is necessary to reproduce temperatures for this species in Florida waters. Coral skeletons sampled using lower resolution methods showed an artificial attenuation of the annual range in skeletal δ18O, with similar δ18O minima during the skeleton represented by the summer months, but larger differences in the winter δ18O maxima. Replicate isotope transects from fast and slow growing areas and different regions of the corallite were also compared. The δ18O of rapidly growing (8 mm/y) portions of the colony was 0.1 to 0.2‰ heavier than the slowest growing (1.1 mm/y) portions of the colony. This difference as well as the difference between the skeleton sampled at high and low resolutions appears to result in part from the attenuation of the δ18O signal as a result of the reduced sampling rate in slower growing sections of the coral and is not solely a result of variable kinetic effects

Cadmium isotopes in Bahamas platform carbonates:A base for reconstruction of past surface water bioproductivity and their link with chromium isotopes

The distribution of cadmium (Cd) within the oceans strongly suggests that it is used as a nutrient by marine phytoplankton. Biologically induced removal of Cd from modern surface waters is accompanied by an isotopic fractionation leaving surface-waters enriched in isotopically heavy Cd. This first study focusses on tying the Cd isotopic record preserved in modern shallow platform carbonates of the Great Bahama Bank (GBB) to conditions in the upper water column, and provides a base for future studies aiming at reconstructing past bioproductivity levels in ancient ocean/basin surface waters. In addition, we compare δ114Cd values with previously published chromium (Cr) isotope values and link signals of bioproductivity with redox conditions in the surface waters. The GBB core samples yield [Cd] (21–188 μg/kg), which increases with depth alongside changes in carbonate mineralogy related to sediment supply and diagenesis. The δ114Cd values of these carbonates are mainly positively fractionated with an average of 0.11‰ ± 0.17 (2σ; n = 17) relative to the NIST 3108 reference standard. Unlike previously observed for Cr isotopes, there is no control of δ114Cd values by relative abundances of the carbonate polymorphs aragonite and calcite in the studied profile. Likewise, δ114Cd values are not correlated to major and trace element (e.g. Ca, Mg, Mn and Sr) contents. We postulate that the burial and diagenetic processes of carbonate cannot modify the Cd isotope signals. Using the experimental fractionation factor for Cd into calcite (−0.45‰), calculated seawater δ114Cd of +0.56 ± 0.17‰ is in agreement with values for modern North Atlantic Surface Seawater. This study's results suggest that δ114Cd values in carbonates are a reliable tool for reconstruction of bioproductivity levels in past surface seawaters, and open new possibilities in combination with Cr isotopes to link these with past ocean redox.[Display omitted]•First study of Cd isotopes as a tracer applied to modern platform carbonates.•Coupling of double tracer Cd—Cr isotopes to bioproductivity in ambient seawater.•Cd isotopes show insensitivity to diagenetic processes and various carbonate phases