Did Photosymbiont Bleaching Lead to the Demise of Planktic Foraminifer Morozovella at the Early Eocene Climatic Optimum?

Valeria Luciani,Roberta D'Onofrio,Gerald R Dickens,Bridget S Wade

doi:10.1002/2017pa003138

Valeria Luciani, Roberta D'Onofrio + Show 2 more

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https://doi.org/10.1002/2017pa003138

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Abstract

The symbiont‐bearing mixed‐layer planktic foraminiferal genera Morozovella and Acarinina were among the most important calcifiers of early Paleogene tropical–subtropical oceans. A marked and permanent switch in the abundance of these genera is known to have occurred at low‐latitude sites at the beginning of the Early Eocene Climatic Optimum (EECO), such that the relative abundance of Morozovella permanently and significantly decreased along with a progressive reduction in the number of species; concomitantly, the genus Acarinina almost doubled its abundance and diversified. Here we examine planktic foraminiferal assemblages and stable isotope compositions of their tests at Ocean Drilling Program Site 1051 (northwest Atlantic) to detail the timing of this biotic event, to document its details at the species level, and to test a potential cause: the loss of photosymbionts (bleaching). We also provide stable isotope measurements of bulk carbonate to refine the stratigraphy at Site 1051 and to determine when changes in Morozovella species composition and their test size occurred. We demonstrate that the switch in Morozovella and Acarinina abundance occurred rapidly and in coincidence with a negative carbon isotope excursion known as the J event (~53 Ma), which marks the start of the EECO. We provide evidence of photosymbiont loss after the J event from a size‐restricted δ13C analysis. However, such inferred bleaching was transitory and also occurred in the acarininids. The geologically rapid switch in planktic foraminiferal genera during the early Eocene was a major evolutionary change within marine biota, but loss of photosymbionts was not the primary causal mechanism.

Highlights

The symbiont-bearing mixed-layer genera Morozovella and Acarinina were among the most important calcifiers of early Paleogene tropical and subtropical oceans (e.g., Aze et al, 2011; Boersma et al, 1987; Pearson et al, 2006; Premoli Silva & Boersma, 1988)
The species M. gracilis, M. lensiformis, M. marginodentata, and M. subbotinae were dominant before the J event, and the large reductions in these four species give rise to the major morozovellid collapse
To understand the striking Morozovella abundance decline near the onset of the Eocene Climatic Optimum (EECO) at low-latitude sites (Frontalini et al, 2016; Luciani et al, 2016), we have refined the stratigraphy at Ocean Drilling Program (ODP) Site 1051 and documented changes in planktic foraminiferal assemblages and their geochemistry

Summary

Introduction

The symbiont-bearing mixed-layer genera Morozovella and Acarinina were among the most important calcifiers of early Paleogene tropical and subtropical oceans (e.g., Aze et al, 2011; Boersma et al, 1987; Pearson et al, 2006; Premoli Silva & Boersma, 1988). Morozovella abundances decreased significantly while Acarinina abundances almost doubled (Luciani et al, 2016) This major turnover further relates to taxonomic diversity, consisting of species reduction among Morozovella and species diversification among Acarinina (Aze et al, 2011; Pearson et al, 2006). This turnover occurred in the temperate Southern Hemisphere setting of Atlantic Site 1263 (Luciani et al, 2017). The timing of the Morozovella–Acarinina switch is interesting because the EECO

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