Did Photosymbiont Bleaching Lead to the Demise of Planktic ForaminiferMorozovellaat the Early Eocene Climatic Optimum?

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 δ 13 C 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.

Demise of the Planktic Foraminifer Genus Morozovella during the Early Eocene Climatic Optimum: New Records from ODP Site 1258 (Demerara Rise, Western Equatorial Atlantic) and Site 1263 (Walvis Ridge, South Atlantic)

“…For counting, we kept specimens of Subbotina senni separate from other subbotinids, because this taxon likely occupied a different habitat [38,41,[62][63][64]. Specifically, S. senni is considered a mixed-layer form that sank to middle mixed-layer or deeper depths during gametogenesis (e.g., [38,63] and references therein).…”

“…If Site 1258 were closer to the CCD prior to the EECO hyperthermals, this would increase its susceptibility to carbonate dissolution for a given carbon input. [41]. The bulk sediment δ 13 C record from Site 1263 is from [42] (blue) while the benthic foraminiferal (N. truempyi) δ 13 C record comes from [19] (grey).…”

“…This includes planktic foraminifera, the tests of which form a common component of many marine sediment records since the Cretaceous. In particular, a major turnover between two dominant genera, Morozovella and Acarinina, occurred at multiple sites (Figure 1) near the onset of the EECO; more specifically, the abundance and diversity of Morozovella decreased significantly while the abundance and diversity of Acarinina increased significantly [19,[36][37][38][39][40][41][42]. At multiple locations in the northern hemisphere with paleo-latitudes of about 30° N, this "switch" happened rapidly and very close to the J event [e.g., 19,[41][42][43], a possible hyperthermal that happened at ∼53.…”

“…The new results provide important insights on the timing and causes of the Morozovella/Acarinina abundance switch and the overall morozovellid species decline. foraminiferal records spanning the Early Eocene Climatic Optimum (EECO) (e.g., [19,41,42]). Base map is from http://www.odsn/de/services/paleomap.html with paleolatitudes modified for Sites 577, 1051 and 1258 according to www.paleolatitude.org model version 1.2 [45].…”

“…Note that locations might be adjusted to a different reference frame to account for changes in plate motion relative to the spin axis [3]. and 1263, and the Possagno section (red dots), which each have bulk carbonate δ 13 C and planktic foraminiferal records spanning the Early Eocene Climatic Optimum (EECO) (e.g., [19,41,42]). Base map is from http://www.odsn/de/services/paleomap.html with paleolatitudes modified for Sites 577, 1051 and 1258 according to www.paleolatitude.org model version 1.2 [45].…”