Carbon isotope (δ13C) records from marine sediments and sedimentary rocks have been extensively used in Cenozoic chemostratigraphy. The early Paleogene interval in particular has received exceptional attention because negative carbon isotope excursions (CIEs) documented in the sedimentary record, for example, at the Paleocene Eocene Thermal Maximum (PETM), ca ∼56 Ma, are believed to reflect significant global carbon cycle perturbations during the warmest interval of the Cenozoic era. However, while bulk carbonate δ13C values exhibit robust correlations across widely separated marine sedimentary basins, their absolute values and magnitude of CIEs vary spatially, especially over time intervals characterized by major deviations in global carbon cycling. Moreover, bulk carbonates in open‐marine environments are an ensemble of different components, each with a distinct isotope composition. Consequently, a comprehensive interpretation of the bulk‐δ13C record requires an understanding of co‐evolution of these components. In this study, we dissect sediments, from the late Paleocene‐early Eocene interval, at ODP Site 1209 (Shatsky Rise, Pacific Ocean) to investigate how a temporally varying bulk carbonate ensemble influences the overall carbon isotope record. A set of 45 samples were examined for δ13C and δ18O compositions, as bulk and individual size fractions. We find a significant increase in coarse‐fraction abundance across the PETM, driven by a changing community structure of calcifiers, modulating the size of the CIE at Site 1209 and thus making it distinct from those recorded at other open‐marine sites. These results highlight the importance of biogeography in the marine stable isotope record, especially when isotope excursions are driven by climate‐ and/or carbon cycle changes. In addition, community composition changes will alter the interpretation of weight percent coarse fraction as proxy for carbonate dissolution.
The Paleocene‐Eocene Thermal Maximum (PETM) is the most pronounced global warming event of the early Paleogene related to atmospheric CO2increases. It is characterized by negative δ18O and δ13C excursions recorded in sedimentary archives and a transient disruption of the marine biosphere. Sites from the U.S. Atlantic Coastal Plain show an additional small, but distinct δ13C excursion below the onset of the PETM, coined the “pre‐onset excursion” (POE), mimicking the PETM‐forced environmental perturbations. This study focuses on the South Dover Bridge core in Maryland, where the Paleocene‐Eocene transition is stratigraphically constrained by calcareous nannoplankton and stable isotope data, and in which the POE is well‐expressed. The site was situated in a middle neritic marine shelf setting near a major outflow of the paleo‐Potomac River system. We generated high‐resolution benthic foraminiferal assemblage, stable isotope, trace‐metal, grain‐size and clay mineralogy data. The resulting stratigraphic subdivision of this Paleocene‐Eocene transition is placed within a depth transect across the paleoshelf, highlighting that the PETM sequence is relatively expanded. The geochemical records provide detailed insights into the paleoenvironment, developing from a well‐oxygenated water column in latest Paleocene to a PETM‐ecosystem under severe biotic stress‐conditions, with shifts in food supply and temperature, and under dysoxic bottom waters in a more river‐dominated setting. Environmental changes started in the latest Paleocene and culminated atthe onset of the PETM, hinting to an intensifying trigger rather than to an instantaneous event at the Paleocene‐Eocene boundary toppling the global system.
more » « less- Award ID(s):
- 2103513
- PAR ID:
- 10472061
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 37
- Issue:
- 10
- ISSN:
- 2572-4517
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
The chronology of the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma) remains disputed, hampering complete understanding of the possible trigger mechanisms of this event. Here we present an astrochronology for the PETM carbon isotope excursion from Howards Tract, Maryland a paleoshelf environment, on the mid-Atlantic Coastal Plain. Statistical evaluation of variations in calcium content and magnetic susceptibility indicates astronomical forcing was involved and the PETM onset lasted about 6 kyr. The astrochronology and Earth system modeling suggest that the PETM onset occurred at an extreme in precession during a maximum in eccentricity, thus favoring high temperatures, indicating that astronomical forcing could have played a role in triggering the event. Ca content data on the paleo-shelf, along with other marine records, support the notion that a carbonate saturation overshoot followedmore » « less
-
more » « less
Abstract The Paleocene‐Eocene Thermal Maximum (PETM; 56 Ma) is considered to be one of the best analogs for future climate change. The carbon isotope composition (δ13C) of
n ‐alkanes derived from leaf waxes of terrestrial plants and marine algae can provide important insights into the carbon cycle perturbation during the PETM. Here, we present new organic geochemical data and compound‐specific δ13C data from sediments recovered from an early Cenozoic basin‐margin succession from Spitsbergen. These samples represent one of the most expanded PETM sites and provide new insights into the high Arctic response to the PETM. Our results reveal a synchronous ∼−6.5‰ carbon isotope excursion (CIE) in short‐chainn ‐alkanes (n C19; marine algae/bacteria) with a ∼−5‰ CIE in long‐chainn ‐alkanes (n C29andn C31; plant waxes) during the peak of the PETM. Although δ13Cn ‐alkanesvalues were potentially affected via a modest thermal effect (1‰–2‰), the relative changes in the δ13Cn ‐alkanesremain robust. A simple carbon cycle modeling suggests peak carbon emission rate could be ∼3 times faster than previously suggested using δ13CTOCrecords. The CIE magnitude of both δ13Cn ‐C19and δ13Cn ‐C29can be explained by the elevated influence of13C‐depleted respired CO2in the water column and increased water availability on land, elevatedp CO2in the atmosphere, and changes in vegetation type during the PETM. The synchronous decline in δ13C of both leaf waxes and marine algae/bacteria argues against a significant contribution to the sedimentary organic carbon pool from the weathering delivery of fossiln ‐alkanes in the Arctic region. -
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.
-
more » « less
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.
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1029/2022PA004475
