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At least 775 records · Page 43Linked to original sources

USGS2013

In situ spectrophotometric measurement of dissolved inorganic carbon in seawater

Autonomous in situ sensors are needed to document the effects of today’s rapid ocean uptake of atmospheric carbon dioxide (e.g., ocean acidification). General environmental conditions (e.g., biofouling, turbidity) and carbon-specific conditions (e.g., wide diel variations) present significant challenges to acquiring long-term measurements of dissolved inorganic carbon (DIC) with satisfactory accuracy and resolution. SEAS-DIC is a new in situ instrument designed to provide calibrated, high-frequency, long-term measurements of DIC in marine and fresh waters. Sample water is first acidified to convert all DIC to carbon dioxide (CO 2 ). The sample and a known reagent solution are then equilibrated across a gas-permeable membrane. Spectrophotometric measurement of reagent pH can thereby determine the sample DIC over a wide dynamic range, with inherent calibration provided by the pH indicator’s molecular characteristics. Field trials indicate that SEAS-DIC performs well in biofouling and turbid waters, with a DIC accuracy and precision of &sim;2 μmol kg –1 and a measurement rate of approximately once per minute. The acidic reagent protects the sensor cell from biofouling, and the gas-permeable membrane excludes particulates from the optical path. This instrument, the first spectrophotometric system capable of automated in situ DIC measurements, positions DIC to become a key parameter for in situ CO 2 -system characterizations.

Environmental Science & Technology↗

USGS2016

Formation of the Isthmus of Panama

The formation of the Isthmus of Panama stands as one of the greatest natural events of the Cenozoic, driving profound biotic transformations on land and in the oceans. Some recent studies suggest that the Isthmus formed many millions of years earlier than the widely recognized age of approximately 3 million years ago (Ma), a result that if true would revolutionize our understanding of environmental, ecological, and evolutionary change across the Americas. To bring clarity to the question of when the Isthmus of Panama formed, we provide an exhaustive review and reanalysis of geological, paleontological, and molecular records. These independent lines of evidence converge upon a cohesive narrative of gradually emerging land and constricting seaways, with formation of the Isthmus of Panama sensu stricto around 2.8 Ma. The evidence used to support an older isthmus is inconclusive, and we caution against the uncritical acceptance of an isthmus before the Pliocene.

Science Advances↗

USGS2023

A simple approach to modeling light attenuation in the Sacramento-San Joaquin Delta using commonly available data

The diffuse attenuation coefficient of photosynthetically active radiation (KdPAR) is commonly used to predict light attenuation in aquatic productivity models, but obtaining measurements of PAR to compute KdPAR is difficult. In situ calculations of KdPAR require multiple measurements of PAR through the water column, and these measurements are infeasible for real-time recording. Instead, predictive models using surface-water measurements may be used. Traditional KdPAR models are based on open-ocean habitats and rely on chlorophyll—as a proxy measurement for phytoplankton abundance—as the main predictive parameter. However, elevated suspended sediments and dissolved organic materials may also affect KdPAR values of inland water bodies and estuaries. In this study, we leverage KdPAR calculations derived from in situ light measurements collected along with surface-water-quality parameters across the Sacramento-San Joaquin River Delta in California, USA (the Delta). Sampling occurred between January of 2013 and May of 2014. We also explored regional and seasonal effects, but these did not clearly affect the model. Ultimately, the best-performing model included surface-level turbidity only (R2 = 0.91). The simplicity of the model facilitates use of KdPAR estimates for a variety of purposes throughout the Delta, including euphotic depth calculations, and as inputs to primary-productivity and habitat-suitability models. We demonstrate the model’s usability with two open-sources data sets (one spatially dense, and one temporally dense), and estimate KdPAR, euphotic depth, and primary productivity within the Delta. We provide calculations for each estimation, allowing users to easily adopt these models and apply them to their own data or with open-sourced data, which are abundant.

California↗

USGS2019

(U-Th)/He zircon dating of Chesapeake Bay distal impact ejecta from ODP site 1073

Single crystal (U‐Th)/He dating has been undertaken on 21 detrital zircon grains extracted from a core sample from Ocean Drilling Project (ODP) site 1073, which is located ~390 km northeast of the center of the Chesapeake Bay impact structure. Optical and electron imaging in combination with energy dispersive X‐ray microanalysis (EDS) of zircon grains from this late Eocene sediment shows clear evidence of shock metamorphism in some zircon grains, which suggests that these shocked zircon crystals are distal ejecta from the formation of the ~40 km diameter Chesapeake Bay impact structure. (U‐Th/He) dates for zircon crystals from this sediment range from 33.49 ± 0.94 to 305.1 ± 8.6 Ma (2σ), implying crystal‐to‐crystal variability in the degree of impact‐related resetting of (U‐Th)/He systematics and a range of different possible sources. The two youngest zircon grains yield an inverse‐variance weighted mean (U‐Th)/He age of 33.99 ± 0.71 Ma (2σ uncertainties n = 2; mean square weighted deviation = 2.6; probability [ P ] = 11%), which is interpreted to be the (U‐Th)/He age of formation of the Chesapeake Bay impact structure. This age is in agreement with K/Ar, 40 Ar/ 39 Ar, and fission track dates for tektites from the North American strewn field, which have been interpreted as associated with the Chesapeake Bay impact event.

Chesapeake Bay↗

USGS2018

Storm surge propagation and flooding in small tidal rivers during events of mixed coastal and fluvial influence

The highly urbanized estuary of San Francisco Bay is an excellent example of a location susceptible to flooding from both coastal and fluvial influences. As part of developing a forecast model that integrates fluvial and oceanic drivers, a case study of the Napa River and its interactions with the San Francisco Bay was performed. For this application we utilize Delft3D-FM, a hydrodynamic model that computes conservation of mass and momentum on a flexible mesh grid, to calculate water levels that account for tidal forcing, storm surge generated by wind and pressure fields, and river flows. We simulated storms with realistic atmospheric pressure, river discharge, and tidal forcing to represent a realistic joint fluvial and coastal storm event. Storm conditions were applied to both a realistic field-scale Napa river drainage as well as an idealized geometry. With these scenarios, we determine how the extent, level, and duration of flooding is dependent on these atmospheric and hydrologic parameters. Unsurprisingly, the model indicates that maximal water levels will occur in a tidal river when high tides, storm surge, and large fluvial discharge events are coincident. Model results also show that large tidal amplitudes diminish storm surge propagation upstream and that phasing between peak fluvial discharges and high tide is important for predicting when and where the highest water levels will occur. The interactions between tides, river discharge, and storm surge are not simple, indicating the need for more integrated flood forecasting models in the future.

California↗

USGS2024

Where ice gave way to fire: Deglacial volcanic activity at the edge of the Coast Mountains in Milbanke Sound, BC

Kitasu Hill and MacGregor Cone formed along the Principe Laredo Fault on British Columbia’s central coast as the Wisconsinan ice sheet withdrew from the Coast Mountains. These small-volume Milbanke Sound Volcanoes (MSV) provide remarkable evidence for the intimate relationship between volcanic and glacial facies. The lavas are within-plate, differentiated (low MgO < 7%) Ocean Island Basalts, hawaiites, and mugearites that formed from ∼1% decompression melting of asthenosphere with residual garnet. Kitasu Hill, on glaciated bedrock, formed between 18 and 15 cal ka BP. Dipping, poorly stratified, admixed hyaloclastite, and glacial diamicton with large plutonic clasts and pillow breccia comprise its basal tuya platform (0–43 masl). Subaerial nested cinder cones, with smaller capping lava flows, sit atop the tuya. New marine samples show McGregor Cone formed subaerially but now sits submerged at 43–200 mbsl on an eroded moraine at the mouth of Finlayson Channel. Seismic data and cores reveal glaciomarine sediments draping the cone’s lower slopes and show beach terraces. Cores contain glaciomarine diamictons, ice-rafted debris, delicate glassy air fall tephra, and shallow, sublittoral, and deeper benthic foraminifera. Dates of 14.1–11.2 cal ka BP show volcanism spanned ∼2000 years during floating ice shelf conditions. The MSV have similar proximal positions to the retreating ice sheet, display mixed volcano-glacial facies, and experienced similar unloading stresses during deglaciation. The MSV may represent deglacially triggered volcanism. The dates, geomorphic and geological evidence, constrain a local relative sea level curve for Milbanke Sound and show how ice gave way to fire.

Milbanke Sound↗

USGS2016

Geochemistry and origin of metamorphosed mafic rocks from the Lower Paleozoic Moretown and Cram Hill Formations of North-Central Vermont: Delamination magmatism in the western New England appalachians

The Moretown Formation, exposed as a north-trending unit that extends from northern Vermont to Connecticut, is located along a critical Appalachian litho-tectonic zone between the paleomargin of Laurentia and accreted oceanic terranes. Remnants of magmatic activity, in part preserved as metamorphosed mafic rocks in the Moretown Formation and the overlying Cram Hill Formation, are a key to further understanding the tectonic history of the northern Appalachians. Field relationships suggest that the metamorphosed mafic rocks might have formed during and after Taconian deformation, which occurred at ca . 470 to 460 Ma. Geochemistry indicates that the sampled metamorphosed mafic rocks were mostly basalts or basaltic andesites. The rocks have moderate TiO 2 contents (1–2.5 wt %), are slightly enriched in the light-rare earth elements relative to the heavy rare earths, and have negative Nb-Ta anomalies in MORB-normalized extended rare earth element diagrams. Their chemistry is similar to compositions of basalts from western Pacific extensional basins near volcanic arcs. The metamorphosed mafic rocks of this study are similar in chemistry to both the pre-Silurian Mount Norris Intrusive Suite of northern Vermont, and also to some of Late Silurian rocks within the Lake Memphremagog Intrusive Suite, particularly the Comerford Intrusive Complex of Vermont and New Hampshire. Both suites may be represented among the samples of this study. The geochemistry of all samples indicates that parental magmas were generated in supra-subduction extensional environments during lithospheric delamination.

American Journal of Science↗

USGS1987

The Japan Trench and its juncture with the Kuril Trench: cruise results of the Kaiko project, Leg 3

This paper presents the results of a detailed survey combining Seabeam mapping, gravity and geomagnetic measurements as well as single-channel seismic reflection observations in the Japan Trench and the juncture with the Kuril Trench during the French-Japanese Kaiko project (northern sector of the Leg 3) on the R/V “Jean Charcot”. The main data acquired during the cruise, such as the Seabeam maps, magnetic anomalies pattern, and preliminary interpretations are discussed. These new data cover an area of 18,000 km 2 and provide for the first time a detailed three-dimensional image of the Japan Trench. Combined with the previous results, the data indicate new structural interpretations. A comparative study of Seabeam morphology, single-channel and reprocessed multichannel records lead to the conclusion that along the northern Japan Trench there is little evidence of accretion but, instead, a tectonic erosion of the overriding plate. The tectonic pattern on the oceanic side of the trench is controlled by the creation of new normal faults parallel to the Japan Trench axis, which is a direct consequence of the downward flexure of the Pacific plate. In addition to these new faults, ancient normal faults trending parallel to the N65° oceanic magnetic anomalies and oblique to the Japan trench axis are reactivated, so that two directions of normal faulting are observed seaward of the Japan Trench. Only one direction of faulting is observed seaward of the Kuril Trench because of the parallelism between the trench axis and the magnetic anomalies. The convergent front of the Kuril Trench is offset left-laterally by 20 km relative to those of the Japan Trench. This transform fault and the lower slope of the southernmost Kuril Trench are represented by very steep scarps more than 2 km high. Slightly south of the juncture, the Erimo Seamount riding on the Pacific plate, is now entering the subduction zone. It has been preceded by at least another seamount as revealed by magnetic anomalies across the landward slope of the trench. Deeper future studies will be necessary to discriminate between the two following hypothesis about the origin of the curvature between both trenches: Is it due to the collision of an already subducted chain of seamounts? or does it correspond to one of the failure lines of the America/Eurasia plate boundary?

Earth and Planetary Science Letters↗

USGS2008

Molecular and structural characterization of dissolved organic matter from the deep ocean by FTICR-MS, including hydrophilic nitrogenous organic molecules

Dissolved organic matter isolated from the deep Atlantic Ocean and fractionated into a so-called hydrophobic (HPO) fraction and a very hydrophilic (HPI) fraction was analyzed for the first time by Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) to resolve the molecular species, to determine their exact masses, and to calculate their molecular formulas. The elemental composition of about 300 molecules was identified. Those in the HPO fraction (14C age of 5100 year) are very similar to much younger freshwater fulvic acids, but less aromatic and more oxygenated molecules are more frequent. This trend continues toward the HPI fraction and may indicate biotic and abiotic aging processes that this material experienced since its primary production thousands of years ago. In the HPI fraction series of nitrogenous molecules containing one, two, or three nitrogens were identified by FTICR-MS. Product ion spectra of the nitrogenous molecules suggest that the nitrogen atoms in these molecules are included in the (alicyclic) backbone of these molecules, possibly in reduced form. These mass spectrometric data suggest that a large set of stable fulvic acids is ubiquitous in all aquatic compartments. Although sources may differ, their actual composition and structure appears to be quite similar and largely independent from their source, because they are the remainder of intensive oxidative degradation processes. ?? 2008 American Chemical Society.

Environmental Science & Technology↗

USGS2016

Evaluation of reproductive status in Atlantic Tripletail by traditional and nonlethal approaches

Reproductive biology information is an important tool for fishery management actions such as the identification of spawning areas and the development of protective size limits, bag limits, and seasons. Such information for the management of Atlantic Tripletail Lobotes surinamensis is currently limited, particularly in the western Atlantic Ocean, as information regarding the reproductive biology of this species is sparse in the published literature. To this end, we determined the reproductive status of tripletail and compared the results of a nonlethal sampling method, plasma vitellogenin (VTG) analysis, with those of two traditional (lethal) methods, gonadosomatic index (GSI) and gonad histology. A total of 223 (122 male and 101 female) tripletail were sampled over 2 years near Jekyll Island, Georgia. Gonad histology indicated that 107 (94%) of the male tripletail were in the spawning-capable reproductive phase. Female tripletail were found in all reproductive phases, but only nine (8.9%) were in the spawning-capable phase. Plasma VTG was strongly related to GSI in females ( R 2 = 0.832, n = 77), and female GSI differed significantly among reproductive phases ( p < 0.0001). The estimated length at which 50% (L50) of female tripletail reached maturity was 463 mm; however, the L50 for male tripletail could not be determined because of the lack of immature fish within the study sample. Our study provides valuable information for the management of tripletail and indicates that a nonlethal approach (plasma VTG) may be useful for differentiating developing and spawning-capable females from males and from females in other reproductive phases.

Georgia↗

USGS2023

Coupling large-spatial scale larval dispersal modelling with barcoding to refine the amphi-Atlantic connectivity hypothesis in deep-sea seep mussels

In highly fragmented and relatively stable cold-seep ecosystems, species are expected to exhibit high migration rates and long-distance dispersal of long-lived pelagic larvae to maintain genetic integrity over their range. Accordingly, several species inhabiting cold seeps are widely distributed across the whole Atlantic Ocean, with low genetic divergence between metapopulations on both sides of the Atlantic Equatorial Belt (AEB, i.e. Barbados and African/European margins). Two hypotheses may explain such patterns: (i) the occurrence of present-day gene flow or (ii) incomplete lineage sorting due to large population sizes and low mutation rates. Here, we evaluated the first hypothesis using the cold seep mussels Gigantidas childressi, G. mauritanicus, Bathymodiolus heckerae and B. boomerang . We combined COI barcoding of 763 individuals with VIKING20X larval dispersal modelling at a large spatial scale not previously investigated. Population genetics supported the parallel evolution of Gigantidas and Bathymodiolus genera in the Atlantic Ocean and the occurrence of a 1-3 Million-year-old vicariance effect that isolated populations across the Caribbean Sea. Both population genetics and larval dispersal modelling suggested that contemporary gene flow and larval exchanges are possible across the AEB and the Caribbean Sea, although probably rare. When occurring, larval flow was eastward (AEB - only for B. boomerang ) or northward (Caribbean Sea - only for G. mauritanicus ). Caution is nevertheless required since we focused on only one mitochondrial gene, which may underestimate gene flow if a genetic barrier exists. Non-negligible genetic differentiation occurred between Barbados and African populations, so we could not discount the incomplete lineage sorting hypothesis. Larval dispersal modelling simulations supported the genetic findings along the American coast with high amounts of larval flow between the Gulf of Mexico (GoM) and the US Atlantic Margin, although the Blake Ridge population of B. heckerae appeared genetically differentiated. Overall, our results suggest that additional studies using nuclear genetic markers and population genomics approaches are needed to clarify the evolutionary history of the Atlantic bathymodioline mussels and to distinguish between ongoing and past processes.

Frontiers in Marine Science↗

USGS2025

Molluscan isotope sclerochronology in marine palaeoclimatology: Taxa, technique and timespan issues

Study of the accretionary biomineralised hardparts of organisms (sclerochronology) can make a useful contribution to palaeoclimatology. Ontogenetic sequences of isotopic data (δ 18 O and Δ 47 values) from the shells of marine molluscs are a source of information on seasonal sea-surface temperatures that can be used for detailed and rigorous evaluation of the outputs of numerical climate models. In situations where there is significant seasonality, and where shell preservation is adequate, accurate information about winter and summer surface temperature can be obtained from shallow-water benthic forms (bivalves and gastropods), in particular the early ontogeny of fast-growing species. Accurate information about winter surface temperature can also be obtained from individuals that lived at mid-shelf depths (20–40 m), but summer seafloor values from these need upward adjustment to derive a plausible surface temperature if thermal stratification of the water column occurs in this season. Ontogenetic δ 18 O profiles from planktonic pteropod gastropods are a potential source of insight into seasonal surface temperatures in the ocean basins; these organisms merit investigation for provision of information to complement shelf data. Temperature profiles constructed from shell δ 18 O require an estimate of the δ 18 O value of ambient seawater, which can be derived by back-calculation from the Δ 47 -temperature supplied by the same shell material. Alternatively, through appropriate sampling and data processing, seasonal temperatures can be obtained directly from Δ 47 profiles. Climate parameters are defined in terms of the mean state over a period of 30 consecutive years, a statistic (e.g., for seasonal temperatures) which can be derived from the long isotopic temperature records obtainable from bivalve species that live for many tens or hundreds of years. Efforts should be made to acquire such records, especially averaged data from crossdated shells, to specify climate parameters for precise times in the past. Information for precise times would be of particular value for icehouse intervals like the late Cenozoic, characterised by high frequency (high amplitude) climate fluctuation. Short records from non-crossdated shells can nevertheless provide useful insights into climate, particularly if a large dataset is obtained, supplying a reliable picture of the mean state and range of variation in climate parameters over the interval represented by the shells.

Quaternary Science Reviews↗

USGS2013

The PRISM (Pliocene Palaeoclimate) reconstruction: Time for a paradigm shift

Global palaeoclimate reconstructions have been invaluable to our understanding of the causes and effects of climate change, but single-temperature representations of the oceanic mixed layer for data–model comparisons are outdated, and the time for a paradigm shift in marine palaeoclimate reconstruction is overdue. The new paradigm in marine palaeoclimate reconstruction stems the loss of valuable climate information and instead presents a holistic and nuanced interpretation of multi-dimensional oceanographic processes and responses. A wealth of environmental information is hidden within the US Geological Survey's P liocene R esearch, I nterpretation and S ynoptic M apping (PRISM) marine palaeoclimate reconstruction, and we introduce here a plan to incorporate all valuable climate data into the next generation of PRISM products. Beyond the global approach and focus, we plan to incorporate regional climate dynamics with emphasis on processes, integrating multiple environmental proxies wherever available in order to better characterize the mixed layer, and developing a finer time slice within the Mid-Piacenzian Age of the Pliocene, complemented by underused proxies that offer snapshots into environmental conditions. The result will be a proxy-rich, temporally nested, process-oriented approach in a digital format - a relational database with geographic information system capabilities comprising a three-dimensional grid representing the surface layer, with a plethora of data in each cell.

Philosophical Transactions of the Royal Society A:↗

USGS2020

Mercury export from Arctic great rivers

Land–ocean linkages are strong across the circumpolar north, where the Arctic Ocean accounts for 1% of the global ocean volume and receives more than 10% of the global river discharge. Yet estimates of Arctic riverine mercury (Hg) export constrained from direct Hg measurements remain sparse. Here, we report results from a coordinated, year-round sampling program that focused on the six major Arctic rivers to establish a contemporary (2012–2017) benchmark of riverine Hg export. We determine that the six major Arctic rivers exported an average of 20 000 kg y –1 of total Hg (THg, all forms of Hg). Upscaled to the pan-Arctic, we estimate THg flux of 37 000 kg y –1 . More than 90% of THg flux occurred during peak river discharge in spring and summer. Normalizing fluxes to watershed area (yield) reveals higher THg yields in regions where greater denudation likely enhances Hg mobilization. River discharge, suspended sediment, and dissolved organic carbon predicted THg concentration with moderate fidelity, while suspended sediment and water yields predicted THg yield with high fidelity. These findings establish a benchmark in the face of rapid Arctic warming and an intensifying hydrologic cycle, which will likely accelerate Hg cycling in tandem with changing inputs from thawing permafrost and industrial activity.

Environmental Science & Technology↗

USGS2021

New faunal records from a World Heritage Site in danger: Rennell Island, Solomon Islands

Remote oceanic islands have high potential to harbor unique fauna and flora, but opportunities to conduct in-depth biotic surveys are often limited. Furthermore, underrepresentation of existing biodiversity in the literature has the potential to detract from conservation planning and action. Between 18 and 29 October 2018, we surveyed the terrestrial vertebrates of East Rennell, a UNESCO World Heritage Site in Solomon Islands. We documented 56 species, including 15 squamates, 13 mammals, and 38 birds, and present four new vertebrate records for the island: Stephan's emerald dove ( Chalcophaps stephani ), Maluku myotis ( Myotis moluccarum ), littoral skink ( Emoia atrocostata ) and brahminy blindsnake ( Indotyphlops braminus ). East Rennell was designated a World Heritage site for its significant on-going ecological and biological processes, and importance for the study of island biogeography. The new records presented here provide evidence that continued field studies combined with DNA analysis will continue to uncover even greater endemic biodiversity. Rennell is currently experiencing major habitat destruction in parts of the island that are not under World Heritage protection, and we anticipate collateral damage will likely extend into protected areas. Our survey also underscores the incredible vertebrate biodiversity that stands to be lost unless conservation actions and local community needs are intertwined to promote beneficial outcomes on both fronts.

Pacific Science↗

USGS1996

The nature, distribution, and origin of gas hydrate in the Chile Triple Junction region

A bottom simulating reflector (BSR) is regionally distributed throughout much of the Chile Triple Junction (CTJ) region. Downhole temperature and logging data collected during Ocean Drilling Program (ODP) Leg 141 suggest that the seismic BSR is generated by low seismic velocities associated with the presence of a few percent free gas in a ??? 10 m thick zone just beneath the hydrate-bearing zone. The data also indicate that the temperature and pressure at the BSR best corresponds to the seawater/methane hydrate stability field. The origin of the large amounts of methane required to generate the hydrates is, however, problematic. Low total organic carbon contents and low alkalinities argue against significant in situ biogenic methanogenesis, but additional input from thermogenic sources also appears to be precluded. Increasing thermal gradients, associated with the approach of the spreading ridge system, may have caused the base of the hydrate stability field to migrate 300 m upwards in the sediments. We propose that the upward migration of the base of the stability field has concentrated originally widely dispersed hydrate patches into the more continuous hydrate body we see today. The methane can be concentrated if the gas hydrates can form from dissolved methane, transported into the hydrate zone via diffusion or fluid advection. A strong gradient may exist in dissolved methane concentration across the BSR leading to the steady reabsorbtion of the free gas zone during the upward migration of the BSR even in the absence of fluid advection.

Earth and Planetary Science Letters↗

USGS1985

An estimate of hydrothermal fluid residence times and vent chimney growth rates based on 210Pb Pb ratios and mineralogic studies of sulfides dredged from the Juan de Fuca Ridge

The 210 PbPb ratios across two sulfide samples dredged from the Juan de Fuca Ridge are used to estimate the growth rate of the sulfide material and the residence time of the hydrothermal fluid within the oceanic crust from the onset of basalt alteration. 210 Pb is added to the hydrothermal fluid by two processes: (1) high-temperature alteration of basalt and (2) if the residence time of the fluid is on the order of the 22.3-year half-life of 210 Pb, by in-situ growth from 222 Rn ( Krishnaswami and Turekian, 1982 ). Stable lead is derived only from the alteration of basalt. The 210 Pb/Pb ratio across one sample was ∼ 0.5 dpm/10 −6 g Pb, and across the other it was ∼ 0.4 dpm/10 −6 g Pb. These values are quite close to the 238 UP/b ratios of basalts from the area, suggesting that the residence time of the hydrothermal fluid from the onset of basalt alteration is appreciably less than the mean life of 210 Pb, i.e., the time required for ingrowth from the radon. An apparent growth rate of 1.2 cm/yr is derived from the slope of the 210 Pb/Pb curve for one of the samples. This is consistent with its mineralogy and texture which suggest an accretionary pattern of development. There is no obvious sequential growth pattern, and virtually no gradient in 210 Pb/Pb across the second sample. This is consistent with alteration of the original 210 Pb/Pb distribution by extensive remobilization reactions which are inferred from the mineralogic and textural relationships of the sample.

Earth and Planetary Science Letters↗

USGS2016

Interpreting the role of pH on stable isotopes in large benthic foraminifera

Large benthic foraminifera (LBF) are prolific producers of calcium carbonate sediments in shallow, tropical environments that are being influenced by ocean acidification (OA). Two LBF species, Amphistegina gibbosa (Order Rotaliida) with low-Mg calcite tests and Archaias angulatus (Order Miliolida) with high-Mg calcite tests, were studied to assess the effects of pH 7.6 on oxygen and carbon isotopic fractionation between test calcite and ambient seawater. The δ 18 O and δ 13 C values of terminal chambers and of whole adult tests of both species after 6 weeks were not significantly different between pH treatments of 8.0 and 7.6. However, tests of juveniles produced during the 6-week treatments showed significant differences between δ 18 O and δ 13 C values from control (pH 8.0) when compared with the treatment (pH 7.6) for both species. Although each individual's growth was photographed and measured, difficulty in distinguishing and manually extracting newly precipitated calcite from adult specimens likely confounded any differences in isotopic signals. However, juvenile specimens that resulted from asexual reproduction that occurred during the experiments did not contain old carbonate that could confound the new isotopic signals. These data reveal a potential bias in the design of OA experiments if only adults are used to investigate changes in test chemistries. Furthermore, the results reaffirm that different calcification mechanisms in these two foraminiferal orders control the fractionation of stable isotopes in the tests and will reflect decreasing pH in seawater somewhat differently. .

ICES Journal of Marine Science↗