An expanded database of Southern Hemisphere surface sediment dinoflagellate cyst assemblages and their oceanographic affinities

Abstract

Dinoflagellate cyst assemblages present a valuable proxy to infer paleoceanographic conditions, yet factors influencing geographic distributions of species remain largely unknown, especially in the Southern Ocean. Strong lateral transport, sea-ice dynamics and a low and uneven geographic coverage of surface sediment samples have limited the use of dinocyst assemblages as a quantitative proxy for paleo-environmental conditions such as sea-surface temperature (SST), nutrient concentrations, salinity and sea ice (presence). In this study we present a new set of surface sediment samples (n=66) from around Antarctica, doubling the number of Antarctic-proximal samples to 100 (dataset wsi_100) and increasing the total number of Southern Hemisphere samples to 655 (dataset sh_655). Additionally, we use modelled ocean conditions and apply Lagrangian techniques on all SouthernHemisphere sample stations to quantify and evaluate the influence of lateral transport on the sinking trajectory of microplankton and through that, to the inferred ocean conditions. K-means cluster analysis on the wsi_100 dataset demonstrates the strong affinity of Selenopemphix antarctica to sea-ice cover conditions, and Islandinium spp. to low-salinity conditions. For the entire Southern Hemisphere, the k-means cluster analysis identifies nine clusters with a characteristic assemblage. In most clusters a single dinocyst species dominates the assemblage. These clusters correspond to well-defined oceanic conditions in specific Southern Ocean zones or along the ocean fronts. We find that when lateral transport is predominantly zonal, the environmental parameters inferred from the sea floor assemblages mostly correspond to those of overlying ocean surface. In this case, the transport factor can thus be neglected and will not represent a bias in the reconstructions. Yet, for some individual sites, e.g., deep water sites or sites under strong current regimes, lateral transport can play a large role. The results of our study further constrain environmental conditions represented by dinocyst assemblages and the location of Southern Ocean frontal systems.

Reconciling equatorward migration of Southern Ocean fronts with minor ice volume change during Miocene cooling

“…From ~7 Ma, N. labyrinthus started to decrease in abundance. The Nlab-Cluster and the slightly warmer high-Ocen-Cluster alternate in the Pliocene (5-2 Ma) on orbital timescales, which is close to the modern assemblage, and bracket the modern STF 42 (SSTs between 25 °C and 10 °C; Fig. 5a).…”

“…This assemblage resembles the Spin cluster of ref 42 , which now mainly thrives along the northwest coast of Australia and in low latitudes in the eastern Indian Ocean 42 . This cluster is associated with a modern SST of ~29±0.5°C, a temperature in line with that derived from biomarkers 27,42 (Fig. 5a).…”

“…2). This species (Nlab cluster in ref 42 ) is found most abundant in sediments south of the STF, in the modern subantarctic zone. We interpret the proliferation of Nlab and a progressive cooling towards subantarctic zone-like conditions (Fig.…”

“…2), and GDGT concentrations are low 27 , pointing to enhanced sediment oxidation. The available palynological data for the MCO shows that the Spin cluster was replaced by Impagidinium paradoxum and I. patulum, which in the modern are restricted to temperate to equatorial open ocean regions between sub-tropical and subpolar systems 42 . Although it is unclear how this dinocyst assemblage differs from the Spin cluster in terms of ocean temperature, the biomarker-based SSTs indicate continued warmth during the MCO at the site (Fig.…”

“…To shed light on the links between (Antarctic) ice volume and dynamics, Southern Ocean oceanography and latitudinal SST gradients, we present a detailed reconstruction of Neogene STF migration history and surface and bottom water temperature offshore Tasmania, and pair these with new estimates of Antarctic ice volume change from the MCO across the MMCT. We use dinoflagellate cyst (dinocyst) biogeography 42 to reconstruct the position of Southern Ocean currents and fronts and combine these with published SST reconstructions.…”