Combined Effects of Gametogenic Calcification and Dissolution on δ 18 O Measurements of the Planktic Foraminifer Trilobatus sacculifer

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 19; no. 11; pp. 4487 - 4501
• Main Authors: Wycech, Jody B., Kelly, Daniel Clay, Kitajima, Kouki, Kozdon, Reinhard, Orland, Ian J., Valley, John W.
• Format: Journal Article
• Language: English
• Published: 01.11.2018
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2018GC007908

Abstract Oxygen isotope ratios (δ 18 O) measured from planktic foraminifer shells are commonly used to reconstruct past surface ocean conditions, yet the shells of many planktic foraminifers are an aggregate mixture of multiple carbonate phases with differing δ 18 O compositions. Here we demonstrate how secondary ion mass spectrometry can be used to measure intrashell δ 18 O heterogeneity by performing in situ analyses on micrometer‐scale (3–10 μm) domains within individual shells of the extant, mixed layer species Trilobatus sacculifer . Secondary ion mass spectrometry measurements on shells taken from Holocene‐aged sediments at three sites in different ocean basins confirm that the δ 18 O of gametogenic (GAM) calcite added to shells during the terminal (reproductive) stage of the life cycle is 1.0–1.4‰ higher than that of pregametogenic calcite. Examination of shells in cross section reveals that many have suffered varying degrees of internal dissolution, which further skews whole‐shell δ 18 O compositions toward higher values by preferentially removing low δ 18 O pregametogenic calcite. The results of this study echo the calls of earlier studies cautioning that spatiotemporal changes in the proportion of high δ 18 O GAM calcite should be considered when assessing T .  sacculifer δ 18 O records generated via conventional isotope ratio mass spectrometry. Plain Language Summary Foraminifera are unicellular marine organisms that grow microscopic shells made of the mineral calcite. Foraminifer shells are commonly preserved in deep‐sea sediments, and their whole‐shell chemistries are analyzed to reconstruct the history of ocean climate change. However, these analyses can be problematic because some foraminifera add multiple calcite phases with differing chemistries during their life cycle. Here we use a novel analytical technique called secondary ion mass spectrometry to measure oxygen isotope ratios on minute spots within individual shells of the planktic foraminifer, Trilobatus sacculifer . Our results show that whole T .  sacculifer shells are a mixture of two geochemically distinct calcite phases—one with low oxygen isotope ratios that formed near the warm sea surface and one with high oxygen isotope ratios that formed as the shells sink into colder water. Hence, addition of secondary calcite in deeper water elevates whole‐shell oxygen isotope values and introduces inaccuracies into sea surface reconstructions derived from conventional measurements. Images of the shells in cross section reveal that this inaccuracy in whole‐shell measurements is compounded by the preferential dissolution of the internal calcite. Our study illustrates potential issues with whole‐shell oxygen isotope measurements and demonstrates how secondary ion mass spectrometry analysis can improve the fidelity of sea surface reconstructions. Key Points Secondary ion mass spectrometry confirms gametogenic calcite has higher δ 18 O values than pregametogenic calcite in Trilobatus sacculifer Dissolution preferentially removes pregametogenic calcite without altering surface texture of shells Dissolution biases whole‐shell δ 18 O values toward high‐δ 18 O gametogenic calcite