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March 11 @ 9:00 am - 12:30 pm CST

Student: Joyeeta Bhattacharya
Department: Earth, Environmental and Planetary Sciences
Defense Date: Thursday, March 11, 2021
Time: 9:00 a.m.
Thesis Title: Response of marine carbonate accumulation to carbon cycle perturbations during the hothouse-warmhouse climate of the Eocene

Abstract:

Major climatic transitions and aberrations accompanied by global carbon cycle
perturbations are known to have occurred during the Eocene epoch, between 56-34 Ma. The
warmest interval of the Cenozoic culminated during early Eocene, punctuated by multiple events
of rapid injection of isotopically light carbon into the ocean-atmosphere system, e.g. Paleocene
Eocene Thermal Maximum (PETM), ca. 56 Ma, which were preserved as negative carbon isotope
excursions (CIEs) in the sedimentary record. The hothouse climate of early Eocene transitioned to
a relatively cooler and less explored ‘warmhouse’ regime during middle-late Eocene, when
multiple transient warming events occurred, which were at times associated with significant
fluctuations in the oceanic carbonate saturation horizons. This study documents changes in
carbonate dissolution indices and carbonate mass accumulation rate through the entire Eocene (56–
34 Ma) at ODP Site 1209 on Shatsky Rise, north-central Pacific and ties it to a stable carbon and
oxygen isotope record, which is stratigraphically correlated with the global benthic stack and
equatorial Pacific CCD reconstruction of the Eocene, in order to explore the links between climate
change, carbon cycling and marine carbonate accumulation. Our study finds a strong correlation
between magnitude of CIEs and intensity of dissolution for early Eocene hyperthermal events
which appears to significantly weaken for multiple dissolution events in middle-late Eocene,
possibly indicating fundamental difference in their causal mechanism.

As this study and multiple others in paleoceanography utilize bulk sediment carbon isotope
(δ13C) records in Cenozoic chemostratigraphy; it is crucial to deconvolute the isotope signal of
marine bulk sediments. Pelagic carbonates are ensemble of different components of varying
proportion and distinct isotope composition; hence spatial and temporal co-variation of pelagic
carbonate constituents is important to understand in order to make a comprehensive interpretation
of bulk-δ13C records. In this part of the study, sediments from the late Paleocene-early Eocene
interval (58-50 Ma), at ODP Site 1209 are dissected into size fractions to investigate how a
temporally varying bulk carbonate ensemble influences the overall carbon isotope record. A set of
45 samples were examined for their bulk and size-fraction-specific δ13C and δ18O compositions.
We find a significant increase in coarse-fraction abundance across PETM, driven by a changing
community structure of calcifiers, modulating the size of 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 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 a conventional proxy for carbonate dissolution.
The next part of the work focusses on chemostratigraphy of the Lutetian-Bartonian stages
of middle Eocene from sites on Tasman Sea, in the southwest Pacific, which represent a
‘warmhouse’ climate state: a transition from supergreenhouse/hothouse climate of early Eocene to
the onset of polar glaciation and ‘coolhouse’ climate of Oligocene. Despite long-term cooling
conditions in the background, middle Eocene was punctuated by at least one significant global
warming and sea-floor carbonate dissolution event, referred to as the Middle Eocene Climate
Optimum (MECO), ca. ~40.5 Ma. Over the last decade, studies from the Atlantic deep ocean sites
have identified at least one other brief event of carbon cycle perturbation, during polarity chron
19R; referred to as Late Lutetian Thermal Maximum (LLTM). But its global extent remains
unresolved. Our current work develops a stable isotope stratigraphy of middle Eocene sediments,
precisely within the late Lutetian – early Bartonian stages from four different sites across Tasman
Sea, two of which were drilled during IODP Expedition 371 (U1508C and U1509A) and two of
which derive from field work in New Caledonia (the Sommet-Khian outcrop and the Cadart drill
core). Here we identify long-term changes in carbon and oxygen isotope records, possibly related
to 405 Ky eccentricity cycles; and present a δ13C-stratigraphic framework for the Tasman sea
geochemical record in context of the global benthic stack. We also document the occurrence of
LLTM (both in open- and marginal marine setting), which constitutes its first evidence from the
Pacific, and hence corroborates its globally widespread occurrence. Also, we find the occurrence
of MECO at Site U1509, consistent with its expression in the Southern Ocean. A positive carbon
and oxygen isotope excursion along with emergence of pure carbonate system is documented in
New Caledonia within NP16 biozone of middle Eocene; however, its global relevance remains
elusive because of lack of reliable age datum. Ultimately this study provides evidence for a
dynamic climate in middle Eocene and shows that marine sedimentary carbon isotopes constitute
a robust and reliable tool for Cenozoic chemostratigraphic correlation.

Details

Date:
March 11
Time:
9:00 am - 12:30 pm CST
Event Categories:
,

Venue

Dept. Earth Science, Rice University, 6100 Main Street
Houston, TX 77005 United States
Phone:
7133484880

Details

Date:
March 11
Time:
9:00 am - 12:30 pm CST
Event Categories:
,

Venue

Dept. Earth Science, Rice University, 6100 Main Street
Houston, TX 77005 United States
Phone:
7133484880

For outside visitors, the best way to get to our department is to come in on Rice Blvd and turn left into entrance 20 (intersection of Rice and Kent St.). At the stop sign, you will see a visitor parking lot on your right.  From there, walk east to the department.  The google map below shows exactly where our building is.