Cd/Ca ratios of in situ collected planktonic foraminiferal tests

Abstract

[1] The Cd/Ca ratios of planktonic foraminiferal tests have been used to reconstruct surface water nutrient utilization and paleoproductivity. The reliability of this proxy has hitherto not been comprehensively studied, however. To fill this gap, we present novel Cd/Ca data for in situ sampled and sedimentary planktonic foraminifers of the species Globigerinoides ruber, G. sacculifer, Globigerina bulloides, Orbulina universa, and Globorotalia truncatulinoides from the Arabian Sea and the North Atlantic. The Cd/Ca ratios obtained for G. ruber sampled from the live habitat generally display a correlation with seawater phosphate content, but no such trend is observed for G. sacculifer. This distinct behavior may reflect different ecological niches or speciesspecific incorporation of Cd into the calcite shells of the organisms. The Cd/Ca ratios of G. ruber, G. sacculifer, and G. bulloides from surface sediments are consistently higher than those obtained for live collected specimens of the same species. Postdepositional alteration of the tests is unlikely to be responsible for these systematic differences. Rather, they appear to reflect a combination of factors, including the formation of calcite crusts with high Cd contents, the different timescales that are represented by in situ and sedimentary foraminiferal tests, and the dominance of tests from periods of high productivity in sediments. Our results also reveal higher Cd/Ca ratios for live G. ruber than for settling tests of the same species. This suggests that planktonic foraminiferal shells are partially dissolved while they individually settle through the water column. Sedimentary tests, however, will be less affected by dissolution processes because these shells are primarily deposited in mass sinking events, which feature much higher settling velocities than those experienced by single settling shells.

Silicate Weathering and Carbon Cycle Controls on the Oligocene‐Miocene Transition Glaciation

“…Enrichment of Cd in uncleaned foraminifera may result from recrystallization ( D Cd of inorganic calcite is high; Table ) or incorporation of pore water Cd in Fe‐Mn coatings (Tachikawa & Elderfield, ). Cd/Ca values measured in this study are, however, similar to North Atlantic core top measurements for Globorotalia truncatulinoides (which is also deeper‐thermocline dwelling; Cd/Ca = 0.08 μmol/mol; Ripperger et al, ). This perhaps suggests that our Cd/Ca values are minimally affected by diagenesis although we acknowledge that overprinting of the primary surface water Cd/Ca signal cannot be fully discounted.…”

“… Note . Data from D Li (Delaney et al, ; Hathorne & James, ; Marriott, Henderson, Belshaw, et al, ; Marriott, Henderson, Crompton, et al, ), D Mg (Anand et al, ; Baker et al, ; Dekens et al, ; Lea et al, ; Sexton et al, ), D Cd (Delaney, ; Havach et al, ; Lorens, ; Mashiotta et al, ; McCorkle et al, ; Ripperger et al, ; Rosenthal et al, ), and D U (Meece & Benninger, ; Russell et al, , ).…”

“…Higher D Cd values are common for subthermocline dwellers (e.g., Gr. truncatulinoides ; Ripperger et al, ), and so this D Cd value is arguably more applicable to the lower thermocline dwelling D. venezuelana . Hence, assuming [Ca] sw = 12 mmol/kg and D Cd = 4.1, we estimate that [PO 4 ] increased by ~0.5 μmol/kg during the Mi‐1 event (Figure b).…”

“…The δ 18 O value of seawater varies as a function of salinity and global ice volume. Although unlikely to be constant, for simplicity, the δ 18 O value of seawater for late Oligocene in equation (3) is considered to be À0.5‰ (Delaney et al, 1985;Hathorne & James, 2006;Marriott, Henderson, Belshaw, et al, 2004;Marriott, Henderson, Crompton, et al, 2004), D Mg (Anand et al, 2003;Baker et al, 1982;Dekens et al, 2002;Lea et al, 2000;Sexton et al, 2006), D Cd (Delaney, 1989;Havach et al, 2001;Lorens, 1981;Mashiotta et al, 1997;McCorkle et al, 1995;Ripperger et al, 2008;Rosenthal et al, 1997), and D U (Meece & Benninger, 1993;Russell et al, 1994Russell et al, , 2004.…”

Investigating the fidelity of Mg/Ca and other elemental data from reductively cleaned planktonic foraminifera

“…For Cd/Ca, the offset between the species evolved from higher Cd/Ca in G. sacculifer to nearly the same value of N. dutertrei. The final G. sacculifer Cd/Ca is similar to the value of shells collected from plankton tows [Ripperger et al, 2008]. If the distribution coefficients for these two species are similar, the closer values for the Cd/Ca of the two species are more consistent with expected values based on the water column relationship of these two species and the nutrient distribution in the upper water column in this region [Elderfield and Rickaby, 2000] (http://iridl.ldeo.…”