Depth dependent variation in the Paleocene-Eocene boundary carbon isotope excursion horizon in the N. Pacific

Author(s): Zachos, James C.; Tabor, Maris; Bralower, Timothy J.; Premoli-Silva, Isabella; Malone, Mitchell; Roehl, Ursula
Ocean Drilling Program, Leg 198, Shipboard Scientific Party, College Station, TX
Author Affiliation(s): Primary:
University of California at Santa Cruz, Department of Earth Sciences, Santa Cruz, CA, United States
Other:
University of North Carolina, United States
Universita di Milano, Italy
Ocean Drilling Program, United States
Universität Bremen, Federal Republic of Germany
Volume Title: Geological Society of America, 2002 annual meeting
Source: Abstracts with Programs - Geological Society of America, 34(6), p.461; Geological Society of America, 2002 annual meeting, Denver, CO, Oct. 27-30, 2002. Publisher: Geological Society of America (GSA), Boulder, CO, United States. ISSN: 0016-7592 CODEN: GAAPBC
Note: In English
Summary: The Paleocene-Eocene Thermal Maximum (PETM; a.k.a. LPTM; 55 Mya) was characterized by an abrupt, large negative carbon isotope excursion (CIE) of 2.5 to 5.0 per mil in both marine and continental carbon reservoirs. One hypothesis attributes this isotopic aberration to the rapid dissociation of 1200 to 2000 Gt of marine methane hydrate. In this case, most of the methane (δ13C = -60 per mil), assuming complete oxidation, would have dissolved in the ocean as CO2. Numerical simulations show that that in addition to lowering mean ocean δ13CDIC, the rapid (<103 to 104 y) dissolution of this large mass of CO2 lowers seawater pH and [CO3], thereby triggering a rapid shoaling of the carbonate compensation depth (CCD) and widespread dissolution of seafloor carbonates. Complete restoration of the CCD (to pre-excursion levels) would occur within ∼150 kyr, primarily through dissolution of silicates (on land) and the subsequent deposition of carbonates. To test this hypothesis, Ocean Drilling Program Leg 198 drilled 4 sites (1209-1212) in a depth-transect between 2.4 and 3.1 km on the flanks of Shatsky Rise in the north Pacific. The P-E boundary was recovered within a sequence of carbonate rich ooze at each site. High resolution (∼ every 3 to 10 cm) bulk stable C and O-isotope stratigraphies were constructed for 3 m long sections spanning the boundary at each site. Pronounced isotope excursions occur in each record coincident with the benthic foraminifera extinction horizon, an excursion taxa acme, and a carbonate "dissolution" interval. The δ13C excursions vary in magnitude from 2.5 to 3.0 per mil, and are abrupt occurring in less than a few cm. Recovery is gradual spanning as much as 75 cm. The δ18O records are characterized by negative excursions of ∼0.5 per mil consistent with a ∼2°C warming. The thickness of the C-isotope recovery interval decreases with depth, and is actually step-like at the deepest site, 1211, indicating a brief depositional hiatus. The spatial and temporal patterns of change is consistent with a gradual recovery of the CCD. These findings, together with existing evidence of coeval carbonate dissolution in the deep Atlantic, demonstrate this horizon to be global in extent, thereby supporting the methane dissociation mechanism as the primary cause of the PETM.
Year of Publication: 2002
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 12 Stratigraphy, Historical Geology and Paleoecology; Aliphatic hydrocarbons; Alkanes; Bicarbonate ion; C-13/C-12; Carbon; Carbon dioxide; Cenozoic; Eocene; Gas hydrates; Hydrocarbons; Isotope ratios; Isotopes; Leg 198; Lower Eocene; Methane; North Pacific; Northwest Pacific; O-18/O-16; Ocean Drilling Program; Organic compounds; Oxygen; PH; Pacific Ocean; Paleocene; Paleogene; Shatsky Rise; Solutes; Solution; Stable isotopes; Stratigraphic boundary; Tertiary; Upper Paleocene; West Pacific
Coordinates: N313400 N374800 E1624600 E1571500
Record ID: 2005009090
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