Organic-walled dinoflagellate cysts from the Eocene-Oligocene Transition in the Gulf of Mexico: indicators of climate- and sea-level change during the onset of Antarctic glaciation

The Eocene – Oligocene Transition (EOT, ∼34–33.5 Ma) marks a major transition in Cenozoic climate evolution through the relatively rapid establishment of a continental-scale ice sheet on Antarctica. The EOT is characterized by two ∼200 kyr spaced shifts (termed EOT-1 and Oi-1) in the oxygen isotopic composition (σ18O) of benthic foraminifera, representing both changes in continental ice-volume and temperature. Estimates of the timing and magnitude of these changes during this critical phase in Earth’s climatic evolution are controversial. Here we present marine palynological assemblage data, in particular of organic-walled dinoflagellate cysts (dinocysts), across a classic upper Eocene to lower Oligocene neritic succession cored in Alabama, USA; the Saint Stephens Quarry (SSQ) borehole. These palynological data combined with lithological information allow the identification of three sequence boundaries across the EOT. Critically, we identify a sequence boundary at the level corresponding to the EOT-1. Integrated sea level and paleotemperature records show that EOT-1 primarily represents cooling with some minor and transient continental ice sheet expansion. Furthermore, we identify a significant hiatus, likely caused by major sea level fall at the base of Magnetochron C13 n that corresponds to the Oi-1 shift. This clarifies the σ18O records from SSQ that essentially lack the expected pronounced shift to positive σ18O values so characteristic for Oi-1. Furthermore, we document originations and extinctions of potentially temperature-sensitive dinocysts associated with the EOT-1. In contrast, the Oi-1 does not stand out as period of substantial dinoflagellate turnover. The combined results illustrate that major cooling, limited and transient ice growth and major biotic change were occurring before the full-size expansion of the Antarctic cryosphere

Pentadinium alabamensis: A new, unusual dinoflagellate from the early Oligocene of the Gulf Coast, Alabama, USA

The Eocene–Oligocene Transition (EOT, ~ 34 Ma) marks the onset of major Antarctic ice sheets. The environmental consequences of the transition included major changes in e.g., sea level, temperature, and ocean circulation, complicating biostratigraphic correlations in this interval. Organic walled dinoflagellate cysts (dinocysts) however do show potential for EOT biostratigraphy, especially for ancient shallow marine settings. At St. Stephens Quarry, Alabama, USA, we found anew, extremely suturocavate dinocyst, Pentadinium alabamensis sp. nov., described herein. The range of the new species spans the critical EOT magnetosubchron C13n, making this taxon a useful biostratigraphic marker for this interval in the GulfCoast region. The species appears to be associated with shallow marine, euryhaline condition

Climate aberrations during the middle Miocene: evidence from the eastern North Atlantic Ocean

During the Middle Miocene Climatic Optimum (MMCO; 17-14.5 Ma) the relatively warm climate of the Miocene reached peak temperatures. After the MMCO, the global climate started cooling through several short-lived cooling events, represented by positive oxygen isotope excursions: the Mi-events (Miocene isotope events). One of the more severe events, Mi-3, is associated with East Antarctic Ice Sheet growth and potential Northern Hemisphere ice expansion, as well as marine and terrestrial species turnover and aridification. The causes and consequences of the Mievents are not well constrained yet. CO2 reconstructions combined with the abovementioned consequences suggest that a drawdown of CO2 and/or changes in ocean led to the changes surrounding Mi-3. A minimum node in both eccentricity and obliquity amplitude modulation, an orbital configuration creating favourable conditions for ice growth, has been suggested as a possible triggering mechanism as well. However, an exact cause cannot be pinpointed yet and more high-resolution records are needed in order to investigate the impact and order of events surrounding the Mi-events. Integrated Ocean Drilling Program (IODP) Leg 307 recovered such a high resolution record from the middle Miocene of the Porcupine Basin (offshore south-western Ireland). We have analyzed well-preserved palynomorphs (mainly organic-walled dinoflagellate cysts, acritarchs and pollen) and organic molecules for paleothermometry (e.g. TEX86 and UK’ 37) from site U1318. With these proxies, the development of the Mi-3 event and following Mi-4 have been reconstructed in high resolution (ca. 13 kyr), by assessing e.g. temperature, sea level, thermocline depth and productivity. A pronounced cooling can be observed at Mi-3, and to a lesser degree in Mi-4 as well, together with a sea-level fall and a turnover in the dinocyst record. Our findings also include indications of aridification and a change in wind patterns during Mi-3. This confirms the dramatic impact of the Mi- 3 event, not only on the North Atlantic Ocean, but also on the continent

Environmental- and sea-level change revealed by dinoflagellate cysts during the Eocene-Oligocene transition at St. Stephens Quarry, Alabama, USA

The Eocene-Oligocene transition (EOT, ~34 Myr ago) represents the final transition from the early Paleogene “Greenhouse” into the present “Icehouse” by the initiation of Antarctic glaciation. The EOT is recorded in deep-sea benthic foraminiferal oxygen isotope (δ18O) records as two increasing steps, ~200 kyr apart. However, the relative contribution of cooling and increasing ice-volume cannot be separated from such δ18O records. Independent temperature- and sea-level reconstructions are crucial for understanding the order of events enveloping the onset of Antarctic glaciation. The classic reference section for the EOT, St Stephens Quarry (SSQ) in Alabama, USA, contains a relatively expanded and complete shelf succession. Previous studies at SSQ have already provided benthic foraminiferal stable isotope- and Mg/Ca based temperature information. Sea surface temperatures were reconstructed using TEX86 and planktonic Mg/Ca analyses. Altogether, these data show that the first step of the EOT (precursor or EOT-1) primarily reflects cooling, whereas the second step (or Oi-1) primarily reflects increasing ice-volume. Here, we report on biotic change revealed by evaluating assemblages of fossil remains of organic walled dinoflagellates (dinocysts). Dinoflagellates are a group of unicellular surface dwelling algae and are often used to sensitively record environmental changes. We have inferred sea level change by evaluating dinocyst assemblages in the relatively shallow section of SSQ. This led us to revise the sequence stratigraphy and age model for SSQ. We document a minor sea-level fall associated with the EOT-1 and a more substantial sea-level fall at the Oi-1. At the EOT-1 we furthermore recorded the occurrence of a taxon typically associated with cold water. This is in accordance with the geochemically reconstructed temperature drop of 4–6°C. Early Oligocene assemblages above the Oi-1 are indicative of more productive and shallower lagoonal settings. Our records show that the EOT was a period of profound environmental change, also in the (sub)tropic

Multiproxy record of abrupt sea-surface cooling across the Eocene-Oligocene transition in the Gulf of Mexico

The Eocene-Oligocene transition (EOT; ca. 33–34 Ma) was a time of pronounced climatic change, marked by the establishment of continental-scale Antarctic ice sheets. The timing and extent of temperature change associated with the EOT is controversial. Here we present multiproxy EOT climate records (∼15–34 k.y. resolution) from St. Stephens Quarry, Alabama, USA, derived from foraminiferal Mg/Ca, δ18O, and TEX86. We constrain sea-surface temperatures (SSTs) in the latest Eocene and early Oligocene and address the issue of climatic cooling during the EOT. Paleotemperatures derived from planktic foraminifera Mg/Ca and TEX86 are remarkably consistent and indicate late Eocene subtropical SSTs of >28 °C. There was substantial and accelerated cooling of SSTs (3–4 °C) through the latest Eocene “precursor” δ18O shift (EOT-1), prior to Oligocene Isotope-1 (Oi-1). Our multispecies planktic foraminiferal δ18O records diverge at the E/O boundary (33.7 Ma), signifying enhanced seasonality in the earliest Oligocene in the Gulf of Mexic