Surface-water density variability across the Eocene-Oligocene transition at Falkland Plateau, south Atlantic Ocean
Inferred from process length variability of Operculodinium centrocarpum dinoflagellate cysts Jolien van der Krogt, Frida Snilstveit Hoem, Appy Sluijs, Peter K. Bijl
Marine Palynology and Paleoceanography, Department of Earth Sciences, Utrecht University, Utrecht, the Netherlands. email: p.k.bijl@uu.nl
Introduction
The Eocene-Oligocene transtion marks the onset of continental-scale Antarctic glaciation. While the root cause for this glaciation is though to be a decline of atmospheric CO2 concentrations, favouring amplified positive feedback mechanisms that drove outward growth of the ice sheet, little is known about the region- al oceanographic consequences of Antarctic glaciation, and the importance of those. We here study dino- flagellate cysts from an ocean sediment core from the Falkland Plateau, close to the subtropical front today, and further South during Eocene-Oligocene times. Using a novel dinoflagellate cyst-based proxy, we reconstruct surface-water density (~salinity) conditions across the EOT.
Results
Oi-1a
Oi-1
Take Home Message
- EOT sees sw density increase: T effect? Subtropical front migration?
- Oi-1 and Oi-1a are associated with increased surface-water density, in absence of local T cooling: effect of salinity change?
- Palynological assemblages suggest frontal system migration, no runoff changes - Gradual Oligocene SW freshening: effect of ice berg flux increase?
- Process length changes across study interval large compared to modern calibration
surface water density + --
The toolbox
Operculodinium process length
A; 0%
B; 5%
C; 12.5%
D; >20%
cyst diameter
process length
- Operculodinium common in most samples
- 4 categories (A-D) % pro- cess length of cyst diameter - Avg. process length per cat- egory yields mean process length for each sample
- Qualitative changes in sur- face water density
- Trends through EOT com- pared to TEX86-based SST
Fig. 5. Process lengths for Operculodinium per category: mean and SD, and down-core relative abundance.
Oi-1a
The Site
Deep Sea Drilling Program Site 511 is locat- ed on the Falkland Plateau (Fig. 1). Calcium carbonate stable isotope records have rec- ognized the Oi-1 at ~100 mbsf (Fig. 2). The drilled sequence is part of extensive drift
deposits that formed around the structural highs in the Falkland region.
Fig. 2. Thermocline (Subb.; red) δ18O from DSDP Site 511. dashed line represents the Eocene-Oligocene boundary (figure from Houben et al., 2011).
32 33 34 35 36
0 20 40
60 80 100 120 140
160 180 200
Age (Ma, Gradstein et al. 2004)
FO M. escutiana FCO M. escutiana
FOA M. escutiana
2R 3R 4R 5R 6R 7R 8R 9R 10R 11R 12R 13R 14R 15R 16R 17R 18R 20R
Depth (mbsf)
Core
1
2 3 4
5
6
0 1 2 3 7
į18Osubbotina (‰ VPDB) Oi-1
EOT-1?
Approximate E/O-boundary
Oi1a
Outlook
- Increase temporal resolution
- Direct quantification of process lengths (w/o categories) - Pair results with qualitative proxies for terrestrial runoff (terrestrial/marine ratios, BIT index) to evaluate fe source
Fig. 1. Present-day summer density around Site 511. Ter- restrial freshwater does not reach Site 511 today. Surface water (SW) density increases towards the South because of strong T decrease. Warmer SSTs made SW density lower during EOT, with increasing density at EOT cooling. From:
Verleye et al., 2012 Marine Mi- cropaleontology
The modern system
Fig. 2. Present-day Summer fresh-water flux (FWF). The Falklands were at EOT times more to the southeast wrt Antarctica, so more into the path of the FWF. Higher pre- cipitation over Antarctica
likely favoured iceberg dis- charge, but warmer oceans likely melted them more easily during EOT.
19 20 21 22 23 24 25 26 27
3 5 7 9 11 13
-3 Annual Sea Water Density (+1000 kg m)
Average process length (µm)
Vancouver Island Fjords (this study)
CM3
Fig. 3. Modern calibration curve for Operculodinium cyst process length and sur- face-water density.
Shorter processes higher density= 511
511
Modern surface water density @ Site 511
Merino et al., 2016 Ocean modelling
Gurdebeke et al., 2016 Marine Micropaleontology
no cooling no cooling
Ocea ice
@uu_oceanice
