Geology topics

Howard J. Spero

Publications and source records attributed to Howard J. Spero.

3 recordsLinked to original sources

USGS2023

Comparison of δ13C analyses of individual foraminifer (Orbulina universa) shells by secondary ion mass spectrometry and gas source mass spectrometry

Rationale: The use of secondary ion mass spectrometry (SIMS) to perform micrometer-scale in situ carbon isotope (δ 13 C) analyses of shells of marine microfossils called planktic foraminifers holds promise to explore calcification and ecological processes. The potential of this technique, however, cannot be realized without comparison to traditional whole-shell δ 13 C values measured by gas source mass spectrometry (GSMS). Methods: Paired SIMS and GSMS δ 13 C values measured from final chamber fragments of the same shell of the planktic foraminifer Orbulina universa are compared. The SIMS–GSMS δ 13 C differences (Δ 13 C SIMS-GSMS ) were determined via paired analysis of hydrogen peroxide-cleaned fragments of modern cultured specimens and of fossil specimens from deep-sea sediments that were either untreated, sonicated, and cleaned with hydrogen peroxide or vacuum roasted. After treatment, fragments were analyzed by a CAMECA IMS 1280 SIMS instrument and either a ThermoScientific MAT-253 or a Fisons Optima isotope ratio mass spectrometer (GSMS). Results: Paired analyses of cleaned fragments of cultured specimens ( n = 7) yield no SIMS–GSMS δ 13 C difference. However, paired analyses of untreated ( n = 18) and cleaned ( n = 12) fragments of fossil shells yield average Δ 13 C SIMS-GSMS values of 0.8‰ and 0.6‰ (±0.2‰, 2 SE), respectively, while vacuum roasting of fossil shell fragments ( n = 11) removes the SIMS–GSMS δ 13 C difference. Conclusions: The noted Δ 13 C SIMS-GSMS values are most likely due to matrix effects causing sample–standard mismatch for SIMS analyses but may also be a combination of other factors such as SIMS measurement of chemically bound water. The volume of material analyzed via SIMS is ~10 5 times smaller than that analyzed by GSMS; hence, the extent to which these Δ 13 C SIMS-GSMS values represent differences in analyte or instrument factors remains unclear.

Rapid Communications in Mass Spectrometry↗

USGS2022

Barium enrichment in the non-spinose planktic foraminifer, Globorotalia truncatulinoides

Observations of elevated barium-to-calcium ratios (Ba/Ca) in Globorotalia truncatulinoides have been attributed to contaminant phases, deep calcification depth and diagenetic processes. Here we investigate intra- and inter-test Ba/Ca variability in the non-spinose planktic foraminifer, G. truncatulinoides , from a sediment trap time series in the northern Gulf of Mexico to gain insights into the environmental influences on barium enrichment in this and other non-spinose species. We use laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to differentiate between the elemental composition of the crust and lamellar calcite in non-encrusted (<150 m calcification depth) and encrusted (>150 m calcification depth) specimens of G. truncatulinoides . We find that the Ba/Ca ratio in lamellar calcite is between two and three orders of magnitude higher (10–280 μmol/mol) than that of the crust (0–3 μmol/mol). We include seasonal water column profiles of the Ba/Ca ratio in the northern Gulf of Mexico and determine that the vertical gradient in seawater barium concentration cannot account for the intra-test Ba/Ca variations in G. truncatulinoides . We find the Ba/Ca ratio of the crust to be within the range observed in co-occurring spinose species of foraminifera (pink and white chromotypes of Globigerinoides ruber , and Orbulina universa ) while the range of Ba/Ca in lamellar calcite is consistent with co-occurring non-spinose foraminifera ( Pulleniatina obliquiloculata , Globorotalia menardii , G. tumida , and Neogloboquadrina dutertrei ). Our data are consistent with the hypothesis that G. truncatulinoides calcifies in a marine snow aggregate microenvironment that is enriched in barium relative to ambient seawater. We suggest that G. truncatulinoides crust is formed after the rhizopodia retract and the foraminifer detaches from its marine snow substrate.

Geochimica et Cosmochimica Acta↗

USGS2018

Environmental controls on the geochemistry of Globorotalia truncatulinoides in the Gulf of Mexico: Implications for paleoceanographic reconstructions

Modern observations of planktic foraminifera from sediment trap studies help to constrain the regional ecology of paleoceanographically valuable species. Results from a weekly-resolved sediment trap time series (2008–2014) in the northern Gulf of Mexico demonstrate that 92% of Globorotalia truncatulinoides flux occurs in winter (January, February, and March), and that encrusted and non-encrusted individuals represent calcification in distinct depth habitats. We use individual foraminiferal analysis (IFA) of G. truncatulinoides tests to investigate differences in the elemental (Mg/Ca) and isotopic composition (δ 18 O and δ 13 C) of the encrusted and non-encrusted ontogenetic forms of G. truncatulinoides , and to estimate their calcification depth in the northern Gulf of Mexico. We estimate that non-encrusted and encrusted G. truncatulinoides have mean calcification depths of 66 ± 9 m and 379 ± 76 m, respectively. We validate the Mg/Ca-calcification temperature relationship for G. truncatulinoides and demonstrate that the δ 18 O and Mg/Ca of the non-encrusted form is a suitable proxy for winter surface mixed layer conditions in the Gulf of Mexico. Care should be taken not to combine encrusted and non-encrusted individuals of G. truncatulinoides for down core paleoceanographic studies.

Marine Micropaleontology↗