Spero Howard J.

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Howard J.

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  • Preprint
    Global-mean marine δ13C and its uncertainty in a glacial state estimate
    ( 2015-08) Gebbie, Geoffrey A. ; Peterson, Carlye D. ; Lisiecki, Lorraine E. ; Spero, Howard J.
    A paleo-data compilation with 492 δ13C and δ18O observations provides the opportunity to better sample the Last Glacial Maximum (LGM) and infer its global properties, such as the mean δ13C of dissolved inorganic carbon. Here, the paleocompilation is used to reconstruct a steady-state water-mass distribution for the LGM, that in turn is used to map the data onto a 3D global grid. A global-mean marine δ13C value and a self-consistent uncertainty estimate are derived using the framework of state estimation (i.e., combining a numerical model and observations). The LGM global-mean δ13C is estimated to be 0:14h±0:20h at the two standard error level, giving a glacial-to-modern change of 0:32h±0:20h. The magnitude of the error bar is attributed to the uncertain glacial ocean circulation and the lack of observational constraints in the Pacific, Indian, and Southern Oceans. Observations in the Indian and Pacific Oceans generally have 10 times the weight of an Atlantic point in the computation of the global mean. To halve the error bar, roughly four times more observations are needed, although strategic sampling may reduce this number. If dynamical constraints can be used to better characterize the LGM circulation, the error bar can also be reduced to 0:05 to 0:1h, emphasizing that knowledge of the circulation is vital to accurately map δ13CDIC in three dimensions.
  • Article
    Calibration of the carbon isotope composition (δ13C) of benthic foraminifera
    (John Wiley & Sons, 2017-06-03) Schmittner, Andreas ; Bostock, Helen ; Cartapanis, olivier ; Curry, William B. ; Filipsson, Helena L. ; Galbraith, Eric D. ; Gottschalk, Julia ; Herguera, Juan Carlos ; Hoogakker, Babette ; Jaccard, Samuel L. ; Lisiecki, Lorraine E. ; Lund, David C. ; Martínez Méndez, Gema ; Lynch-Stieglitz, Jean ; Mackensen, Andreas ; Michel, Elisabeth ; Mix, Alan C. ; Oppo, Delia W. ; Peterson, Carlye D. ; Repschläger, Janne ; Sikes, Elisabeth L. ; Spero, Howard J. ; Waelbroeck, Claire
    The carbon isotope composition (δ13C) of seawater provides valuable insight on ocean circulation, air-sea exchange, the biological pump, and the global carbon cycle and is reflected by the δ13C of foraminifera tests. Here more than 1700 δ13C observations of the benthic foraminifera genus Cibicides from late Holocene sediments (δ13CCibnat) are compiled and compared with newly updated estimates of the natural (preindustrial) water column δ13C of dissolved inorganic carbon (δ13CDICnat) as part of the international Ocean Circulation and Carbon Cycling (OC3) project. Using selection criteria based on the spatial distance between samples, we find high correlation between δ13CCibnat and δ13CDICnat, confirming earlier work. Regression analyses indicate significant carbonate ion (−2.6 ± 0.4) × 10−3‰/(μmol kg−1) [CO32−] and pressure (−4.9 ± 1.7) × 10−5‰ m−1 (depth) effects, which we use to propose a new global calibration for predicting δ13CDICnat from δ13CCibnat. This calibration is shown to remove some systematic regional biases and decrease errors compared with the one-to-one relationship (δ13CDICnat = δ13CCibnat). However, these effects and the error reductions are relatively small, which suggests that most conclusions from previous studies using a one-to-one relationship remain robust. The remaining standard error of the regression is generally σ ≅ 0.25‰, with larger values found in the southeast Atlantic and Antarctic (σ ≅ 0.4‰) and for species other than Cibicides wuellerstorfi. Discussion of species effects and possible sources of the remaining errors may aid future attempts to improve the use of the benthic δ13C record.
  • Article
    How well would modern-day oceanic property distributions be known with paleoceanographic-like observations?
    (John Wiley & Sons, 2016-04-08) Gebbie, Geoffrey A. ; Streletz, Gregory J. ; Spero, Howard J.
    Compilations of paleoceanographic observations for the deep sea now contain a few hundred points along the oceanic margins, mid-ocean ridges, and bathymetric highs, where seawater conditions are indirectly recorded in the chemistry of buried benthic foraminiferal shells. Here we design an idealized experiment to test our predictive ability to reconstruct modern-day seawater properties by considering paleoceanographic-like data. We attempt to reconstruct the known, modern-day global distributions by using a state estimation method that combines a kinematic tracer transport model with observations that have paleoceanographic characteristics. When a modern-like suite of observations (Θ, practical salinity, seawater δ18O, inline image, PO4, NO3, and O2) is used from the sparse paleolocations, the state estimate is consistent with the withheld data at all depths below 1500 m, suggesting that the observational sparsity can be overcome. Physical features, such as the interbasin gradients in deep inline image and the vertical structure of Atlantic inline image, are accurately reconstructed. The state estimation method extracts useful information from the pointwise observations to infer distributions at the largest oceanic scales (at least 10,000 km horizontally and 1500 m vertically) and outperforms a standard optimal interpolation technique even though neither dynamical constraints nor constraints from surface boundary fluxes are used. When the sparse observations are more realistically restricted to the paleoceanographic proxy observations of δ13C, δ18O, and Cd/Ca, however, the large-scale property distributions are no longer recovered coherently. At least three more water mass tracers are likely needed at the core sites in order to accurately reconstruct the large-scale property distributions of the Last Glacial Maximum.
  • Preprint
    The influence of salinity on Mg/Ca in planktic foraminifers - evidence from cultures, core-top sediments and complementary δ18O
    ( 2013-06) Honisch, Barbel ; Allen, Katherine A. ; Lea, David W. ; Spero, Howard J. ; Eggins, Stephen M. ; Arbuszewski, Jennifer ; deMenocal, Peter B. ; Rosenthal, Yair ; Russell, Ann D. ; Elderfield, Henry
    The Mg/Ca ratio in foraminiferal calcite is one of the principal proxies used for paleoceanographic temperature reconstructions, but recent core-top sediment observations suggest that salinity may exert a significant secondary control on planktic foraminifers. This study compiles new and published laboratory culture experiment data from the planktic foraminifers Orbulina universa, Globigerinoides sacculifer and Globigerinoides ruber, in which salinity was varied but temperature, pH and light were held constant. Combining new data with results from previous culture studies yields a Mg/Ca-sensitivity to salinity of 4.4±2.3%, 4.7±1.2%, and 3.3±1.7% per salinity unit (95% confidence), respectively, for the three foraminifer species studied here. Comparison of these sensitivities with core-top data suggests that the much larger sensitivity (27±4% per salinity unit) derived from Atlantic core-top sediments in previous studies is not a direct effect of salinity. Rather, we suggest that the dissolution correction often applied to Mg/Ca data can lead to significant overestimation of temperatures. We are able to reconcile culture calibrations with core-top observations by combining evidence for seasonal occurrence and latitude-specific habitat depth preferences with corresponding variations in physico-chemical environmental parameters. Although both Mg/Ca and δ18O yield temperature estimates that fall within the bounds of hydrographic observations, discrepancies between the two proxies highlight unresolved challenges with the use of paired Mg/Ca and δ18O analyses to reconstruct paleo-salinity patterns across ocean basins. The first step towards resolving these challenges requires a better spatially and seasonally resolved δ18Osw archive than is currently available. Nonetheless, site-specific reconstructions of salinity change through time may be valid.