Multiple abrupt warming events (“hyperthermals”) punctuated the Early Eocene and were associated with deep-sea temperature increases of 2 to 4 °C, seafloor carbonate dissolution, and negative carbon isotope (δ13C) excursions. Whether hyperthermals were associated with changes in the global ocean overturning circulation is important for understanding their driving mechanisms and feedbacks and for gaining insight into the circulation’s sensitivity to climatic warming. Here, we present high-resolution benthic foraminiferal stable isotope records (δ13C and δ18O) throughout the Early Eocene Climate Optimum (~53.26 to 49.14 Ma) from the deep equatorial and North Atlantic. Combined with existing records from the South Atlantic and Pacific, these indicate consistently amplified δ13C excursion sizes during hyperthermals in the deep equatorial Atlantic. We compare these observations with results from an intermediate complexity Earth system model to demonstrate that this spatial pattern of δ13C excursion size is a predictable consequence of global warming-induced changes in ocean overturning circulation. In our model, transient warming drives the weakening of Southern Ocean-sourced overturning circulation, strengthens Atlantic meridional water mass aging gradients, and amplifies the magnitude of negative δ13C excursions in the equatorial to North Atlantic. Based on model-data consistency, we conclude that Eocene hyperthermals coincided with repeated weakening of the global overturning circulation. Not accounting for ocean circulation impacts on δ13C excursions will lead to incorrect estimates of the magnitude of carbon release driving hyperthermals. Our finding of weakening overturning in response to past transient climatic warming is consistent with predictions of declining Atlantic Ocean overturning strength in our warm future.
Chert, porcelainite, and other siliceous phases are exceptionally common in Atlantic sedimentary records of the early Eocene, but the origins of these facies remain enigmatic. The early Eocene was also the warmest interval of the entire Cenozoic Era, punctuated by numerous discrete warming events termed “hyperthermals,” the largest of which is termed the Paleocene‐Eocene Thermal Maximum (~56 Ma). Here we present new and published lithologic and carbon isotope records of silica‐bearing lower Eocene sediments and suggest a link between the ubiquitous Atlantic cherts of that time period and hyperthermal events. Our data demonstrate that many of these Atlantic siliceous horizons coincide with negative carbon isotope excursions (a hallmark of hyperthermal events), including a previously unrecognized record of the Paleocene‐Eocene Thermal Maximum in the South Atlantic. Hyperthermal‐associated silica burial appears to be focused in the western middle to high latitudes of both the North and South Atlantic, with no association between siliceous facies and hyperthermal events found in the Pacific. We also present a new model of the coupled carbon and silica cycles (LOSiCAR) to demonstrate that enhanced silicate weathering during these events would require a rapid increase in total marine silica burial. Model experiments that include previously suggested transient reversals in the pattern of deep‐ocean circulation during hyperthermals demonstrate that such a mechanism can explain the apparent focusing of elevated silica burial into the Atlantic. This combination—a silicate weathering feedback in response to global warming along with a circulation‐driven focusing of silica burial—represents a new mechanism for the formation of deep‐sea cherts in lower Eocene Atlantic sedimentary records and may be relevant to understanding chert formation in other intervals of Earth history.
more » « less- PAR ID:
- 10459304
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 34
- Issue:
- 2
- ISSN:
- 2572-4517
- Page Range / eLocation ID:
- p. 287-299
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
-
more » « less
Abstract Environmental and biotic responses to early Eocene hyperthermal events in the southwest Pacific are critical for global paleoclimate reconstructions during Cenozoic greenhouse intervals, but detailed multidisciplinary studies are generally missing from this time and location. Eocene carbonate sediments were recovered during International Ocean Discovery Program Expedition 371 at Site U1510 on southern Lord Howe Rise in the Tasman Sea. Part of the Early Eocene Climatic Optimum (EECO; 53.26–49.14 Ma) and superimposed hyperthermal events have been identified based on refined calcareous nannofossil biostratigraphic data and carbon stable isotope records on bulk sediment and benthic foraminifera. Four negative carbon isotope excursions (CIEs) associated with negative oxygen isotope excursions are recognized within the EECO. Comparison with a global compilation of sites indicates these CIEs correlate to the K event (Eocene Thermal Maximum 3), and tentatively to the S, T, and U events. Sediments with a high carbonate content throughout the EECO provide an excellent opportunity to examine these CIEs, as carbonate dissolution often impacts correlative records elsewhere. Benthic foraminifera and calcareous nannoplankton taxa indicative of warm waters are most abundant during the K event, the most prominent hyperthermal of the EECO. Eutrophication of surface waters during the K event did not lead to increased trophic conditions at the seafloor, whereas a coupled response is observed during smaller hyperthermals. The biotic turnover sheds new light on the paleoenvironmental consequences of hyperthermal events.
-
Abstract. Eocene transient global warming events (hyperthermals) can provide insight into a future warmer world. While much research has focused on the Paleocene–Eocene Thermal Maximum (PETM), hyperthermals of a smaller magnitude can be used to characterize climatic responses over different magnitudes of forcing. This study identifies two events, namely the Eocene Thermal Maximum 2 (ETM2 and H2), in shallow marine sediments of the Eocene-aged Salisbury Embayment of Maryland, based on magnetostratigraphy, calcareous nannofossil, and dinocyst biostratigraphy, as well as the recognition of negative stable carbon isotope excursions (CIEs) in biogenic calcite. We assess local environmental change in the Salisbury Embayment, utilizing clay mineralogy, marine palynology, δ18O of biogenic calcite, and biomarker paleothermometry (TEX86). Paleotemperature proxies show broad agreement between surface water and bottom water temperature changes. However, the timing of the warming does not correspond to the CIE of the ETM2 as expected from other records, and the highest values are observed during H2, suggesting factors in addition to pCO2 forcing have influenced temperature changes in the region. The ETM2 interval exhibits a shift in clay mineralogy from smectite-dominated facies to illite-rich facies, suggesting hydroclimatic changes but with a rather dampened weathering response relative to that of the PETM in the same region. Organic walled dinoflagellate cyst assemblages show large fluctuations throughout the studied section, none of which seem systematically related to CIE warming. These observations are contrary to the typical tight correspondence between climate change and assemblages across the PETM, regionally and globally, and ETM2 in the Arctic Ocean. The data do indicate very warm and (seasonally) stratified conditions, likely salinity-driven, across H2. The absence of evidence for strong perturbations in local hydrology and nutrient supply during ETM2 and H2, compared to the PETM, is consistent with the less extreme forcing and the warmer pre-event baseline, as well as the non-linear response in hydroclimates to greenhouse forcing.more » « less
-
more » « less
Abstract A negative carbon isotope excursion recorded in terrestrial and marine archives reflects massive carbon emissions into the exogenic carbon reservoir during the Paleocene-Eocene Thermal Maximum. Yet, discrepancies in carbon isotope excursion estimates from different sample types lead to substantial uncertainties in the source, scale, and timing of carbon emissions. Here we show that membrane lipids of marine planktonic archaea reliably record both the carbon isotope excursion and surface ocean warming during the Paleocene-Eocene Thermal Maximum. Novel records of the isotopic composition of crenarchaeol constrain the global carbon isotope excursion magnitude to −4.0 ± 0.4‰, consistent with emission of >3000 Pg C from methane hydrate dissociation or >4400 Pg C for scenarios involving emissions from geothermal heating or oxidation of sedimentary organic matter. A pre-onset excursion in the isotopic composition of crenarchaeol and ocean temperature highlights the susceptibility of the late Paleocene carbon cycle to perturbations and suggests that climate instability preceded the Paleocene-Eocene Thermal Maximum.
-
During International Ocean Discovery Program Expeditions 390C, 395E, 390, and 393 (the South Atlantic Transect), seven sites were drilled on the western flank of the southern Mid-Atlantic Ridge. Among these sites, Sites U1557 and U1558 recovered Eocene and Oligocene sediments. Such sediments will allow a better understanding of how ocean ecosystems, as well as ocean circulation and chemistry, responded to the paleoceanographic and paleoclimatic changes leading to the Eocene–Oligocene transition. In this study, we present early Eocene through early Oligocene carbon and oxygen stable isotope data (δ13C and δ18O) of bulk carbonates from sediment samples collected in Holes U1557B, U1558A, and U1558F. The data show that the western South Atlantic, a relatively understudied region for the Eocene, recorded some global geochemical features, such as the relatively low δ13C and δ18O values typical of hyperthermal events characterizing the onset of the Early Eocene Climatic Optimum and the rapid shift toward high δ18O and δ13C values at the Eocene/Oligocene boundary.more » « less
