scholarly journals Equatorial heat accumulation as a long-term trigger of permanent Antarctic ice sheets during the Cenozoic

2016 ◽  
Vol 113 (42) ◽  
pp. 11782-11787 ◽  
Author(s):  
Maxime Tremblin ◽  
Michaël Hermoso ◽  
Fabrice Minoletti
Keyword(s):  
Middle Eocene ◽  
Ice Sheets ◽  
Late Eocene ◽  
Equatorial Atlantic ◽  
Heat Accumulation ◽  
Climate Conditions ◽  
Driving Mechanisms ◽  
Recent Developments ◽  
Antarctic Ice Sheets

Growth of the first permanent Antarctic ice sheets at the Eocene−Oligocene Transition (EOT), ∼33.7 million years ago, indicates a major climate shift within long-term Cenozoic cooling. The driving mechanisms that set the stage for this glaciation event are not well constrained, however, owing to large uncertainties in temperature reconstructions during the Eocene, especially at lower latitudes. To address this deficiency, we used recent developments in coccolith biogeochemistry to reconstruct equatorial Atlantic sea surface temperature (SST) and atmospheric pCO2 values from pelagic sequences preceding and spanning the EOT. We found significantly more variability in equatorial SSTs than previously reported, with pronounced cooling from the Early to Middle Eocene and subsequent warming during the Late Eocene. Thus, we show that the Antarctic glaciation at the Eocene−Oligocene boundary was preceded by a period of heat accumulation in the low latitudes, likely focused in a progressively contracting South Atlantic gyre, which contributed to cooling high-latitude austral regions. This prominent redistribution of heat corresponds to the emplacement of a strong meridional temperature gradient that typifies icehouse climate conditions. Our equatorial coccolith-derived geochemical record thus highlights an important period of global climatic and oceanic upheaval, which began 4 million years before the EOT and, superimposed on a long-term pCO2 decline, drove the Earth system toward a glacial tipping point in the Cenozoic.

scholarly journals Age and driving mechanisms of the Eocene–Oligocene transition from astronomical tuning of a lacustrine record (Rennes Basin, France)

2021 ◽  
Vol 17 (6) ◽  
pp. 2343-2360
Author(s):  
Slah Boulila ◽  
Guillaume Dupont-Nivet ◽  
Bruno Galbrun ◽  
Hugues Bauer ◽  
Jean-Jacques Châteauneuf
Keyword(s):  
Hydrological Cycle ◽  
Sedimentary Facies ◽  
Late Eocene ◽  
Depositional Environments ◽  
Orbital Eccentricity ◽  
Pass Filter ◽  
Low Pass Filter ◽  
Climate Conditions ◽  
Driving Mechanisms ◽  
Natural Gamma

Abstract. The Eocene–Oligocene Transition (EOT) marks the onset of the Antarctic glaciation and the switch from greenhouse to icehouse climates. However, the driving mechanisms and the precise timing of the EOT remain controversial mostly due to the lack of well-dated stratigraphic records, especially in continental environments. Here we present a cyclo-magnetostratigraphic and sedimentological study of a ∼ 7.6 Myr long lacustrine record spanning the late Eocene to the earliest Oligocene, from a drill core in the Rennes Basin (France). Cyclostratigraphic analysis of natural gamma radiation (NGR) log data yields duration estimates of Chrons C12r through C16n.1n, providing additional constraints on the Eocene timescale. Correlations between the orbital eccentricity curve and the 405 kyr tuned NGR time series indicate that 33.71 and 34.10 Ma are the most likely proposed ages of the EO boundary. Additionally, the 405 kyr tuning calibrates the most pronounced NGR cyclicity to a period of ∼1 Myr, matching the g1–g5 eccentricity term, supporting its significant expression in continental depositional environments, and hypothesizing that the paleolake level may have behaved as a low-pass filter for orbital forcing. Two prominent changes in the sedimentary facies were detected across the EOT, which are temporally equivalent to the two main climatic steps, EOT-1 and Oi-1. We suggest that these two facies changes reflect the two major Antarctic cooling/glacial phases via the hydrological cycle, as significant shifts to drier and cooler climate conditions. Finally, the interval spanning the EOT precursor glacial event through EOT-1 is remarkably dominated by obliquity. This suggests preconditioning of the major Antarctic glaciation, either from obliquity directly affecting the formation/(in)stability of the incipient Antarctic Ice Sheet (AIS), or through obliquity modulation of the North Atlantic Deep Water production.

scholarly journals Expansion and diversification of high-latitude radiolarian assemblages in the late Eocene linked to a cooling event in the Southwest Pacific

2015 ◽  
Vol 11 (4) ◽  
pp. 2977-3018 ◽  
Author(s):  
K. M. Pascher ◽  
C. J. Hollis ◽  
S. M. Bohaty ◽  
G. Cortese ◽  
R. M. McKay
Keyword(s):  
Oxygen Isotope ◽  
High Latitude ◽  
Large Scale ◽  
Middle Eocene ◽  
Late Eocene ◽  
Climate History ◽  
Southwest Pacific ◽  
Early Oligocene ◽  
Radiolarian Assemblages

Abstract. The Eocene was characterised by "greenhouse" climate conditions that were gradually terminated by a long-term cooling trend through the middle and late Eocene. This long-term trend was determined by several large-scale climate perturbations that culminated in a shift to "ice-house" climates at the Eocene–Oligocene Transition. Geochemical and micropaleontological proxies suggest that tropical-to-subtropical sea-surface temperatures persisted into the late Eocene in the high-latitude Southwest Pacific Ocean. Here, we present radiolarian microfossil assemblage and foraminiferal oxygen and carbon stable isotope data from Deep Sea Drilling Project (DSDP) Sites 277, 280, 281 and 283 from the middle Eocene to early Oligocene (~ 40–33 Ma) to identify oceanographic changes in the Southwest Pacific across this major transition in Earth's climate history. The Middle Eocene Climatic Optimum at ~ 40 Ma is characterised by a negative shift in foraminiferal oxygen isotope values and a radiolarian assemblage consisting of about 5 % of low latitude taxa Amphicraspedum prolixum group and Amphymenium murrayanum. In the early late Eocene at ~ 37 Ma, a positive oxygen isotope shift can be correlated to the Priabonian Oxygen Isotope Maximum (PrOM) event – a short-lived cooling event recognized throughout the Southern Ocean. Radiolarian abundance, diversity, and preservation increase during the middle of this event at Site 277 at the same time as diatoms. The PrOM and latest Eocene radiolarian assemblages are characterised by abundant high-latitude taxa. These high-latitude taxa also increase in abundance during the late Eocene and early Oligocene at DSDP Sites 280, 281 and 283 and are associated with very high diatom abundance. We therefore infer a~northward expansion of high-latitude radiolarian taxa onto the Campbell Plateau towards the end of the late Eocene. In the early Oligocene (~ 33 Ma) there is an overall decrease in radiolarian abundance and diversity at Site 277, and diatoms are absent. These data indicate that, once the Tasman Gateway was fully open in the early Oligocene, a frontal system similar to the present day was established, with nutrient-depleted subantarctic waters bathing the area around DSDP Site 277, resulting in a more oligotrophic siliceous plankton assemblage.

scholarly journals A 40-million-year history of atmospheric CO 2

2013 ◽  
Vol 371 (2001) ◽  
pp. 20130096 ◽  
Author(s):  
Yi Ge Zhang ◽  
Mark Pagani ◽  
Zhonghui Liu ◽  
Steven M. Bohaty ◽  
Robert DeConto
Keyword(s):  
Middle Miocene ◽  
Carbon Fixation ◽  
Middle Eocene ◽  
Ice Sheet ◽  
Algal Growth ◽  
Equatorial Atlantic ◽  
Antarctic Ice Sheet ◽  
The Past ◽  
Wide Range

The alkenone– p CO 2 methodology has been used to reconstruct the partial pressure of ancient atmospheric carbon dioxide ( p CO 2 ) for the past 45 million years of Earth's history (Middle Eocene to Pleistocene epochs). The present long-term CO 2 record is a composite of data from multiple ocean localities that express a wide range of oceanographic and algal growth conditions that potentially bias CO 2 results. In this study, we present a p CO 2 record spanning the past 40 million years from a single marine locality, Ocean Drilling Program Site 925 located in the western equatorial Atlantic Ocean. The trends and absolute values of our new CO 2 record site are broadly consistent with previously published multi-site alkenone–CO 2 results. However, new p CO 2 estimates for the Middle Miocene are notably higher than published records, with average p CO 2 concentrations in the range of 400–500 ppm. Our results are generally consistent with recent p CO 2 estimates based on boron isotope-pH data and stomatal index records, and suggest that CO 2 levels were highest during a period of global warmth associated with the Middle Miocene Climatic Optimum (17–14 million years ago, Ma), followed by a decline in CO 2 during the Middle Miocene Climate Transition (approx. 14 Ma). Several relationships remain contrary to expectations. For example, benthic foraminiferal δ 18 O records suggest a period of deglaciation and/or high-latitude warming during the latest Oligocene (27–23 Ma) that, based on our results, occurred concurrently with a long-term decrease in CO 2 levels. Additionally, a large positive δ 18 O excursion near the Oligocene–Miocene boundary (the Mi-1 event, approx. 23 Ma), assumed to represent a period of glacial advance and retreat on Antarctica, is difficult to explain by our CO 2 record alone given what is known of Antarctic ice sheet history and the strong hysteresis of the East Antarctic Ice Sheet once it has grown to continental dimensions. We also demonstrate that in the Neogene with low CO 2 levels, algal carbon concentrating mechanisms and spontaneous biocarbonate–CO 2 conversions are likely to play a more important role in algal carbon fixation, which provides a potential bias to the alkenone– p CO 2 method.

Long-term future projections for the Greenland and Antarctic ice sheets with the model SICOPOLIS

2021 ◽  
Author(s):  
Ralf Greve ◽  
Christopher Chambers ◽  
Reinhard Calov ◽  
Takashi Obase ◽  
Fuyuki Saito ◽  
...  
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Ice Sheets ◽  
Future Climate ◽  
Model Intercomparison ◽  
Mass Losses ◽  
Intercomparison Project ◽  
Sensitive Experiment ◽  
Antarctic Ice Sheets

<p>The Coupled Model Intercomparison Project Phase 6 (CMIP6) is a major international climate modelling initiative. As part of it, the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) was devised to assess the likely sea-level-rise contribution from the Greenland and Antarctic ice sheets until the year 2100. This was achieved by defining a set of future climate scenarios by evaluating results of CMIP5 and CMIP6 global climate models (GCMs, including MIROC) over and surrounding the Greenland and Antarctic ice sheets. These scenarios were used as forcings for a variety of ice-sheet models operated by different working groups worldwide (Goelzer et al. 2020, doi: 10.5194/tc-14-3071-2020; Seroussi et al. 2020, doi: 10.5194/tc-14-3033-2020).</p><p>Here, we use the model SICOPOLIS to carry out extended versions of the ISMIP6 future climate experiments for the Greenland and Antarctic ice sheets until the year 3000. For the atmospheric forcing (anomalies of surface mass balance and temperature) beyond 2100, we sample randomly the ten-year interval 2091-2100, while the oceanic forcing beyond 2100 is kept fixed at 2100 conditions. We conduct experiments for the pessimistic, "business as usual" pathway RCP8.5 (CMIP5) / SSP5-8.5 (CMIP6), and for the optimistic RCP2.6 (CMIP5) / SSP1-2.6 (CMIP6) pathway that represents substantial emissions reductions. For the unforced, constant-climate control runs, both ice sheets are stable until the year 3000. For RCP8.5/SSP5-8.5, they suffer massive mass losses: For Greenland, ~1.7 m SLE (sea-level equivalent) for the 12-experiment mean, and ~3.5 m SLE for the most sensitive experiment. For Antarctica, ~3.3 m SLE for the 14-experiment mean, and ~5.3 m SLE for the most sensitive experiment. For RCP2.6/SSP1-2.6, the mass losses are limited to a two-experiment mean of ~0.26 m SLE for Greenland, and a three-experiment mean of ~0.25 m SLE for Antarctica. Climate-change mitigation during the next decades will therefore be an efficient means for limiting the contribution of the ice sheets to sea-level rise in the long term.</p>

scholarly journals Sensitivity of the Antarctic ice sheets to the warming of marine isotope substage 11c

2021 ◽  
Vol 15 (1) ◽  
pp. 459-478
Author(s):  
Martim Mas e Braga ◽  
Jorge Bernales ◽  
Matthias Prange ◽  
Arjen P. Stroeven ◽  
Irina Rogozhina
Keyword(s):  
Sea Level ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Antarctic Ice Sheet ◽  
Climate Signal ◽  
West Antarctic Ice Sheet ◽  
Climate Conditions ◽  
West Antarctic ◽  
The Antarctic ◽  
Antarctic Ice Sheets

Abstract. Studying the response of the Antarctic ice sheets during periods when climate conditions were similar to the present can provide important insights into current observed changes and help identify natural drivers of ice sheet retreat. In this context, the marine isotope substage 11c (MIS11c) interglacial offers a suitable scenario, given that during its later portion orbital parameters were close to our current interglacial. Ice core data indicate that warmer-than-present temperatures lasted for longer than during other interglacials. However, the response of the Antarctic ice sheets and their contribution to sea level rise remain unclear. We explore the dynamics of the Antarctic ice sheets during this period using a numerical ice sheet model forced by MIS11c climate conditions derived from climate model outputs scaled by three glaciological and one sedimentary proxy records of ice volume. Our results indicate that the East and West Antarctic ice sheets contributed 4.0–8.2 m to the MIS11c sea level rise. In the case of a West Antarctic Ice Sheet collapse, which is the most probable scenario according to far-field sea level reconstructions, the range is reduced to 6.7–8.2 m independently of the choices of external sea level forcing and millennial-scale climate variability. Within this latter range, the main source of uncertainty arises from the sensitivity of the East Antarctic Ice Sheet to a choice of initial ice sheet configuration. We found that the warmer regional climate signal captured by Antarctic ice cores during peak MIS11c is crucial to reproduce the contribution expected from Antarctica during the recorded global sea level highstand. This climate signal translates to a modest threshold of 0.4 ∘C oceanic warming at intermediate depths, which leads to a collapse of the West Antarctic Ice Sheet if sustained for at least 4000 years.

scholarly journals Planktonic foraminiferal biostratigraphy and lithology of the Upper Cretaceous (upper Campanian-Maastrichtian) and Palaeogene succession of the Palmyrides (Syria)

2021 ◽  
Vol 74 (1) ◽  
pp. 21-40
Author(s):  
Vlasta Premec-Fućek ◽  
Morana Hernitz Kučenjak ◽  
Gabrijela Pecimotika ◽  
◽  
Keyword(s):  
Middle Eocene ◽  
Planktonic Foraminifera ◽  
Age Determination ◽  
Late Eocene ◽  
Warm Water ◽  
Subtropical Climate ◽  
Foraminiferal Assemblage ◽  
Climate Conditions ◽  
Early Oligocene ◽  
Upper Campanian

An upper Campanian to upper Oligocene stratigraphic succession has been examined from six deep exploration wells in the Palmyrides area of Syria. Most of the sedimentary succession contains rich and well to moderately preserved planktonic foraminiferal assemblages that enable successful age determination. The upper Campanian and Maastrichtian planktonic fauna is highly diverse with domination of warm water taxa such as Globotruncana aegyptiaca, Gansserina gansseri, Globotruncanella havanensis, Globotruncanita angulata and Pseudotextularia elegans. The most dramatic turnover occurred across the Cretaceous/Palaeocene boundary when most planktonic foraminiferal species became extinct. The oldest Palaeocene planktonic foraminiferal assemblage, rich in the number of specimens, but not very diverse, includes the following species: Eoglobigerina eobulloides, Globanomalina archeocompressa, Chiloguembelina morsei, Woodringina claytonensis and Parasubbotina pseudobulloides. The late Palaeocene is marked by origination of the morozovellids, acarininids and globanomalinids, while the early Eocene is characterized by a tropical assemblage, dominated by muricate species, and by intensive speciation of Acarinina and Subbotina in the latest part. Most of these species continue into the middle Eocene and become a significant component of the planktonic community. The middle Eocene is characterized by intensive speciation and domination of warm water genera such as Acarinina, Morozovelloides, and to a lesser degree Turborotalia, Globigerinatheka and Hantkenina. The middle/late Eocene boundary is marked by double extinction of the last muricate taxa Acarinina mcgowrani and Morozovelloides crasssatus, which indicate a variable climate, water column instability, and loss of surface habitats. In contrast, Turborotalia and Globigerinateheka become more important in the late Eocene. The Eocene/Oligocene boundary is marked by the extinction of most warm water taxa including Turborotalia cerroazulensis group, Hantkenina, Globigerinatheka and some subbotinids. The beginning of the early Oligocene is indicated by the domination of cool water taxa such as Dentoglobigerina, Globorotaloides, Tenuitella and Chiloguembelina. Speciation of the spinose surface dweller Ciperoella ciperoensis group reflects warming in the late Oligocene. The combined observations of lithology with the diversity and composition of planktonic foraminifera assemblages indicate that the Palmyrides area in Syria was a Tethyan bioprovince with a tropical to subtropical climate from the late Campanian to the end of the Eocene with deposition in deep sea environments (upper bathyal to outer shelf). In contrast, Oligocene deposits and their microfossil content suggest temperate to warm climate conditions and sedimentation in middle to inner shelf environments.

Sensitivity of the Antarctic ice sheets to the warming of MIS11c

2021 ◽  
Author(s):  
Martim Mas e Braga ◽  
Jorge Bernales ◽  
Matthias Prange ◽  
Arjen P. Stroeven ◽  
Irina Rogozhina
Keyword(s):  
Sea Level ◽  
Climate Model ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Antarctic Ice Sheet ◽  
West Antarctic Ice Sheet ◽  
Climate Conditions ◽  
West Antarctic ◽  
The Antarctic ◽  
Antarctic Ice Sheets

<p><span><span>The Marine Isotope Substage 11c (MIS11c) interglacial (425 – 395 thousand years before present) is a useful analogue to climate conditions that can be expected in the near future, and can provide insights on the natural response of the Antarctic ice sheets to a moderate, yet long lasting warming period. However, its response to the warming of MIS11c and consequent contribution to global sea level rise still remains unclear. We explore the dynamics of the Antarctic ice sheets during this period using a numerical ice-sheet model forced by MIS11c climate conditions derived from climate model outputs scaled by three ice core and one sedimentary proxy records of ice volume. We identify a tipping point beyond which oceanic warming becomes the dominant forcing of ice-sheet retreat, and where collapse of the West Antarctic Ice Sheet is attained when a threshold of 0.4 </span></span><sup><span><span>o</span></span></sup><span><span>C oceanic warming relative to Pre-Industrial levels is sustained for at least 4 thousand years. Conversely, its eastern counterpart remains relatively stable, as it is mostly grounded above sea level. Our results suggest a total sea level contribution from the East and West Antarctic ice sheets of 4.0 – 8.2 m during MIS11c. In the case of a West Antarctic Ice Sheet collapse, which is the most probable scenario according to far-field sea-level reconstructions, this range is reduced to 6.7 – 8.2 m, and mostly reflects uncertainties regarding the initial configuration of the East Antarctic Ice Sheet. </span></span></p>

scholarly journals Orbitally tuned timescale and astronomical forcing in the middle Eocene to early Oligocene

2014 ◽  
Vol 10 (3) ◽  
pp. 955-973 ◽  
Author(s):  
T. Westerhold ◽  
U. Röhl ◽  
H. Pälike ◽  
R. Wilkens ◽  
P. A. Wilson ◽  
...  
Keyword(s):  
High Resolution ◽  
Middle Eocene ◽  
Late Eocene ◽  
Magnetic Polarity ◽  
Driving Mechanisms ◽  
Early Oligocene ◽  
Composite Section ◽  
Age Model ◽  
Absolute Age ◽  
Geomagnetic Polarity

Abstract. Deciphering the driving mechanisms of Earth system processes, including the climate dynamics expressed as paleoceanographic events, requires a complete, continuous, and high-resolution stratigraphy that is very accurately dated. In this study, a robust astronomically calibrated age model was constructed for the middle Eocene to early Oligocene interval (31–43 Ma) in order to permit more detailed study of the exceptional climatic events that occurred during this time, including the middle Eocene climate optimum and the Eocene–Oligocene transition. A goal of this effort is to accurately date the middle Eocene to early Oligocene composite section cored during the Pacific Equatorial Age Transect (PEAT, IODP Exp. 320/321). The stratigraphic framework for the new timescale is based on the identification of the stable long eccentricity cycle in published and new high-resolution records encompassing bulk and benthic stable isotope, calibrated XRF core scanning, and magnetostratigraphic data from ODP Sites 171B-1052, 189-1172, 199-1218, and 207-1260 as well as IODP Sites 320-U1333, and 320-U1334 spanning magnetic polarity Chrons C12n to C20n. Subsequently orbital tuning of the records to the La2011 orbital solution was conducted. The resulting new timescale revises and refines the existing orbitally tuned age model and the geomagnetic polarity timescale from 31 to 43 Ma. The newly defined absolute age for the Eocene–Oligocene boundary validates the astronomical tuned age of 33.89 Ma identified at the Massignano, Italy, global stratotype section and point. The compilation of geochemical records of climate-controlled variability in sedimentation through the middle-to-late Eocene and early Oligocene demonstrates strong power in the eccentricity band that is readily tuned to the latest astronomical solution. Obliquity driven cyclicity is only apparent during 2.4 myr eccentricity cycle minima around 35.5, 38.3, and 40.1 Ma.

scholarly journals Orbitally tuned time scale and astronomical forcing in the middle Eocene to early Oligocene

2013 ◽  
Vol 9 (6) ◽  
pp. 6635-6682 ◽  
Author(s):  
T. Westerhold ◽  
U. Röhl ◽  
H. Pälike ◽  
R. Wilkens ◽  
P. A. Wilson ◽  
...  
Keyword(s):  
High Resolution ◽  
Time Scale ◽  
Middle Eocene ◽  
Late Eocene ◽  
Magnetic Polarity ◽  
Driving Mechanisms ◽  
Early Oligocene ◽  
Age Model ◽  
Geomagnetic Polarity ◽  
New Time

Abstract. Deciphering the driving mechanisms of Earth system processes, including the climate dynamics expressed as paleoceanographic events, requires a complete, continuous, and high-resolution stratigraphy that is very accurately dated. In this study, we construct a robust astronomically calibrated age model for the middle Eocene to early Oligocene interval (31–43 Ma) in order to permit more detailed study of the exceptional climatic events that occurred during this time, including the Middle Eocene Climate Optimum and the Eocene/Oligocene transition. A goal of this effort is to accurately date the middle Eocene to early Oligocene composite section cored during the Pacific Equatorial Age Transect (PEAT, IODP Exp. 320/321). The stratigraphic framework for the new time scale is based on the identification of the stable long eccentricity cycle in published and new high-resolution records encompassing bulk and benthic stable isotope, calibrated XRF core scanning, and magnetostratigraphic data from ODP Sites 171B-1052, 189-1172, 199-1218, and 207-1260 as well as IODP Sites 320-U1333, and -U1334 spanning magnetic polarity Chrons C12n to C20n. Subsequently we applied orbital tuning of the records to the La2011 orbital solution. The resulting new time scale revises and refines the existing orbitally tuned age model and the Geomagnetic Polarity Time Scale from 31 to 43 Ma. Our newly defined absolute age for the Eocene/Oligocene boundary validates the astronomical tuned age of 33.89 Ma identified at the Massignano (Italy) global stratotype section and point. Our compilation of geochemical records of climate-controlled variability in sedimentation through the middle-to-late Eocene and early Oligocene demonstrates strong power in the eccentricity band that is readily tuned to the latest astronomical solution. Obliquity driven cyclicity is only apparent during very long eccentricity cycle minima around 35.5, 38.3 and 40.1 Ma.

Early Tertiary radiation of marine molluscs and the long-term effects of the Cretaceous-Tertiary extinction

Paleobiology ◽  
1988 ◽  
Vol 14 (1) ◽  
pp. 37-51 ◽  
Author(s):  
Thor A. Hansen
Keyword(s):  
Gulf Coast ◽  
Middle Eocene ◽  
Late Eocene ◽  
Ecological Niches ◽  
Long Term Effects ◽  
Gamma Diversity ◽  
Time Diversity ◽  
Total Diversity ◽  
Radiation Phase

The Cretaceous–Tertiary (K–T) extinction reduced the gamma diversity of molluscs on the U.S. Gulf Coast from over 500 species in the late Maastrichtian to a little over 100 species in the early Danian. Gamma (total) diversity increased in a series of steps that generally tracked temperature, to a high of around 400 species in the late Middle Eocene, at which time diversity declined in the Late Eocene–Oligocene extinctions. The molluscan radiation occurred in at least two distinct phases: 1) an Initial Radiation Phase in which certain families underwent unusually high speciation, apparently filling ecological niches vacated by the extinction, followed by extinction of many of the species in these families in the late Danian; and, 2) a Secondary Radiation Phase where gamma diversity gradually increased and new genera gradually appeared. The fact that the gamma diversity of molluscs did not reach pre-extinction levels before the next extinction in the Late Eocene suggests that molluscan faunas may spend much of their evolutionary time recovering from these extinctions.

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