Photosymbiont associations persisted in planktic foraminifera during early Eocene hyperthermals at Shatsky Rise (Pacific Ocean)

Abstract

Understanding the sensitivity of species-level responses to long-term warming will become increasingly important as we look towards a warmer future. Here, we examine photosymbiont associations in planktic foraminifera at Shatsky Rise (ODP Site 1209, Pacific Ocean) across periods of global warming of differing magnitude and duration. We compare published data from the Paleocene-Eocene Thermal Maximum (PETM; ~55.9 Ma) with data from the less intense Eocene Thermal Maximum 2 (ETM2; ~54.0 Ma), and H2 events (~53.9 Ma). We use a positive relationship between test size and carbon isotope value (size-δ13C) in foraminifera shells as a proxy for photosymbiosis in Morozovella subbotinae and Acarinina soldadoensis, and find no change in photosymbiont associations during the less intense warming events, in contrast with PETM records indicating a shift in symbiosis in A. soldadoensis (but not M. subbotinae). Declines in abundance and differing preservation potential of the asymbiotic species Subbotina roesnaesensis along with sediment mixing likely account for diminished differences in δ13C between symbiotic and asymbiotic species from the PETM and ETM2. We therefore conclude that photosymbiont associations were maintained in both A. soldadoensis and M. subbotinae across ETM2 and H2. Our findings support one or both of the hypotheses that 1) changing symbiotic associations in response to warming during the PETM allowed A. soldadoensis and perhaps other acarininids to thrive through subsequent hyperthermals or 2) some critical environmental threshold value was not reached in these less intense hyperthermals.

Fig 1. δ¹³C relative to size class for each interval analyzed.

Intervals designated as ‘peak’ events are shaded in yellow, orange, and brown for the H2, ETM2, and PETM events. Intervals proceeding and following “peak” intervals represent those considered “pre” and “post” event samples, respectively. Significant (p-value < 0.05) linear fits are shown in each plot with a standard error envelope at the 95% confidence interval. PETM data is from [34].

Fig 2

δ¹⁸O records at Site 1209 from A) A. soldadoensis, B) M. subbotinae, C) S. roesnaesensis, and D) benthic Nuttallides truempyi, along with δ¹³C records from E) A. soldadoensis, F) M. subbotinae, G) S. roesnaesensis, and H) benthic N. truempyi. Data from the period prior to 54.323 Ma is from [34]. Benthic foraminiferal records are from [41] (gray) and this study (red), with species denoted by symbol shape. Planktic foraminifera size fraction is denoted by color. The H2, ETM2, and PETM intervals are designated with yellow, orange, and brown shading respectively. All records follow the chronology of [41].

Fig 3. The slope of δ¹³C relative to size at the genus level for Acarinina (red), Morozovella (green) and Subbotina (blue) species from ODP Site 577, after [19], rescaled to the age model of [41] and shown as open circles with error bars.

The H2, ETM2, and PETM intervals are designated with yellow, orange, and brown shading respectively.

Fig 4

A) δ¹³C relative to size fraction at peak (pink) H2, ETM2, and PETM and pre and post event (yellow and purple respectively) intervals for A. soldadoensis, M. subbotinae, and S. roesnaesensis. Multiple time-slices are binned as “pre” “peak” and “post” event (Fig 1, S1 Fig & S1 Table). Linear fits for each relationship are shown with a standard error envelope. B) The slope of δ¹³C relative to size for A. soldadoensis (red squares), M. subbotinae (green circles), and S. roesnaesensis (blue lines) through time at Site 1209. The H2, ETM2, and PETM intervals are designated with yellow, orange, and brown shading respectively.

Fig 5. The Δδ¹³C of A. soldadoensis (red) and M. subbotinae (green), relative to that of S. roesnaesensis (blue) through time.

Note different horizontal axes (time) across panels. The benthic δ¹³C record from [41] is shown in gray above, and the H2, ETM2, and PETM intervals are designated with yellow, orange, and brown shading respectively.

Fig 6

Relative abundance of planktic foraminifera genera (A) before, within, and after the H2 (yellow), ETM2 (orange) and (B) PETM (brown) events. The percent of fragmented tests is shown in black. PETM abundance data from with fragmentation data shown in (Fig 5B of that publication) [43].