The Impact of the Latest Danian Event on Planktic Foraminiferal Faunas at ODP Site 1210 (Shatsky Rise, Pacific Ocean)

Abstract

The marine ecosystem has been severely disturbed by several transient paleoenvironmental events (<200 kyr duration) during the early Paleogene, of which the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma) was the most prominent. Over the last decade a number of similar events of Paleocene and Eocene age have been discovered. However, relatively little attention has been paid to pre-PETM events, such as the "Latest Danian Event" ("LDE", ~62.18 Ma), specifically from an open ocean perspective. Here we present new foraminiferal isotope (δ13C, δ18O) and faunal data from Ocean Drilling Program (ODP) Site 1210 at Shatsky Rise (Pacific Ocean) in order to reconstruct the prevailing paleoceanographic conditions. The studied five-meter-thick succession covers ~900 kyr and includes the 200-kyr-lasting LDE. All groups surface dwelling, subsurface dwelling and benthic foraminifera show a negative δ13C excursion of >0.6‰, similar in magnitude to the one previously reported from neighboring Site 1209 for benthic foraminifera. δ18O-inferred warming by 1.6 to 2.8°C (0.4-0.7‰ δ18O measured on benthic and planktic foraminiferal tests) of the entire water column accompanies the negative δ13C excursion. A well stratified upper ocean directly before and during the LDE is proposed based on the stable isotope gradients between surface and subsurface dwellers. The gradient is less well developed, but still enhanced after the event. Isotope data are supplemented by comprehensive planktic foraminiferal faunal analyses revealing a dominance of Morozovella species together with Parasubbotina species. Subsurface-dwelling Parasubbotina shows high abundances during the LDE tracing changes in the strength of the isotope gradients and, thus, may indicate optimal living conditions within a well stratified surface ocean for this taxon. In addition, distinct faunal changes are reported like the disappearance of Praemurica species right at the base of the LDE and the continuous replacement of M. praeangulata with M. angulata across the LDE.

Fig 1. Stratigraphy, δ¹³C and δ¹⁸O measurements of benthic and planktic foraminifera on ODP Sites 1209:

δ¹³C (green) and δ¹⁸O (red) in a long-term benthic Nuttallides truempyi record (plotted against rmcd [18]). Period, epoch, calcareous nannofossil, age and planktic foraminiferal biostratigraphy are from [18, 26], isotope data are adopted from [18]. Absolute ages given are based on [GTS 2012].

Fig 2. A) and B) δ¹³C and δ¹⁸O of benthic (green), planktic subsurface (blue) and surface (red) foraminifera in comparison to C) XRF measured Fe counts for chemostratigraphic correlation.

The LDE is marked in grey, stable isotope data can be found in S2 Table.

Fig 3. Pacific centered map of the Shatsky Rise (amended after [39]).

ODP Sites 1209 and 1210 are marked by purple stars. Due to plate tectonic movements, the Shatsky Rise Plateau shifted north-westwards over the last 140 Myr. The blue framed star shows the studied site during the middle Paleocene (~60 Ma).

Fig 4. Sedimentary and other parameters of Site 1210.

The LDE is marked in grey, data available in S4 Table. A) Biostratigraphy and Fe XRF core scanning data overlying the sediment core photo. The two main peaks mark the LDE and are used for stratigraphic correlation. The LDE interval covering the XRF Fe peaks are marked by a blue bar. The misfit between XRF core scanning Fe peaks and lithology is most likely an artifact of core expansion in storage. B) CaCO₃ data. C) Coarse fraction (orange) and fragmentation (black) show opposing trends. D) Percentage of planktic foraminifera [%P] (green) shows only little variation below 100% with minima close to Fe maxima. For C) and D): Data points marked as single symbols represent samples that have been excluded from faunal assemblage analyses due to potential diagenetic alteration. E) The amount of planktic foraminifera per g sediment (PFN [# g^-1], pink) has a minimum close to the first LDE peak and a maximum shortly thereafter, however, the variability of planktic foraminiferal accumulation rates (PFAR, black, [# cm^-2 kyr^-1]) is much lower than the absolute abundance of planktic foraminifera per gram sediment. F) Sedimentation rate according to [24] based on the presented cyclostratigraphy therein. G) Simple diversity (grey) and Shannon H’ diversity (black line with a dark grayish background) both indicate a slight decrease during the LDE interval.

Fig 5. δ¹³C–δ¹⁸O plot of foraminiferal isotope measurements.

Distinct clusters separate epibenthic (Nuttallides truempyi, N. umbonifera and intermediate form), planktic subsurface (e.g. Parasubbotina pseudobulloides/variospira, P. varianta) and surface dwelling taxa (e.g. Morozovella angulata). Besides these, isotopic signatures of other taxa were measured to better understand their depth-habitat and paleoecology. Data available in S2 and S3 Tables.

Fig 6. Scanning electron microscope images of planktic foraminifera.

1: Morozovella angulata, lateral view (Sample 1210A-23-3, 0–1.5 cm); 2: Morozovella aequa, umbilical view (1210A-23-3, 37.5–39 cm); 3: Parasubbotina variospira, umbilical (1210A-23-3, 30–31.5 cm); 4a+4b: Igorina albeari umbilical/lateral (1210A-23-1, 90–92 cm); 5: Praemurica uncinata, umbilical (1210A-23-3, 52.5–54 cm); 6: Globanomalina chapmani, umbilical (1210A-23-1, 85–87 cm); 7: Subbotina triangularis, umbilical (1210A-23-3, 52.5–54 cm).

Fig 7. Species abundance.

Fe XRF core scanning data for chemostratigraphic correlation and species abundance data (percentages). Species are sorted according to the stratigraphic order of their abundance maxima. The LDE is marked in grey, data available in S5 Table.

Fig 8. δ¹³C and δ¹⁸O gradient and faunal development.

Gradient of δ¹³C and δ¹⁸O signals calculated by planktic surface minus subsurface dwelling foraminifera, expressed as Δδ¹⁸O and Δδ¹³C. A four-point moving average has been applied. The core photo and the XRF core scanning Fe counts serve for stratigraphic correlation. In addition results from non-metric multidimensional scaling (NMDS) as a statistical measure of the faunal composition are shown (see also Fig 9). The LDE is marked in grey. Data available in S1 and S2 Tables.

Fig 9. Non-Metric multidimensional Scaling.

Results of multivariate analysis (non-metric multidimensional scaling, NMDS, stress = 0.17) showing differences of faunal communities (species-level) between the event stages. Most obvious is a strict division between pre-LDE (green, >234.45 rmcd, numbers 48–59) and post-LDE fauna (blue, <233.75 rmcd, numbers 1–27), whereas the event fauna (red, numbers 28–47) is positioned between the previous two phases, in a mixed position within post-event samples. Numbers refer to samples numerated from top to bottom (cf. S1 Table). From the fitted environmental vectors, the δ¹³C and δ¹⁸O of subsurface dwelling Parasubbotinids are related to the major NMDS axis controlling the distribution. Only the vectors of δ¹³C and δ¹⁸O of the parasubbotinids are significant at the 95% level.