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| Titel |
A Cretaceous benthic foraminiferal stable isotope compilation: implications for paleoclimate and paleoceanography |
| VerfasserIn |
O. Friedrich, R. D. Norris, J. Erbacher |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250030508
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| Zusammenfassung |
| We produced new stable isotope data sets of Cenomanian to Santonian benthic foraminifera
from the western equatorial Atlantic (ODP Leg 207) and from the tropical Pacific Ocean
(DSDP Sites 305 and 463). Together with literature data our results are compiled into a global
isotope compilation, resulting in a continuous benthic δ18O record for the last 115 Ma. This
compilation shows four main intervals during the Cretaceous: (1) increasing temperatures
before 97 Ma and (2) a subsequent super-greenhouse which are both paralleled by
increasing δ13C values, (3) a long-lasting cooling and decrease in carbon isotopes
(90-78 Ma), and (4) globally similar δ13C and δ18O values after 78 Ma. Increasing
sea-surface temperatures sometimes exceeding 35Ë C are well-known for Intervals 1 and
2. But our compilation shows, that deep-ocean temperatures were significantly
warmer than today, especially in the proto- North Atlantic (20-28Ë C). These high
temperatures are explained by a lack of cold bottom-water formation, the restricted
nature of the North Atlantic, and the formation of warm saline bottom waters that
sporadically were formed within epicontinental seas. The parallel positive trend in δ13C is
believed to reflect massive storage of Corg during Cretaceous black shale formation.
Interestingly, however, δ13C values of the tropical Atlantic show a similar trend
but more negative values. We propose that this reflects a combination of extensive
remineralization of 12C and a long residence time due to the sporadic formation of
warm and saline waters. During the following interval 3, benthic δ18O values of all
ocean basins show similar values. This trend is interpreted to be the result of the
beginning opening of the Equatorial Atlantic Gateway. This deepening and a parallel
reorganization of the oceanic circulation with longitudinal water-mass and heat exchange
may have favoured the observed cooling trend of interval 3. This explanation is
supported by the global decrease in δ13C, proposed to reflect a better connection of
the former restricted North Atlantic that allows the oxidization of the organic-rich
sediments formed in this basin. In contrast to the former intervals, the last 13 Ma of the
Cretaceous are, on a global scale, characterized by similar values for both, oxygen and
carbon isotopes. This is proposed to indicate a full connection between all ocean
basins.
Comparison of the Cretacoeus stable isotope compilation with the one of the Cenozoic
reveals the presence of three long-term (~30 Ma) cycles. Each of these cycles is
characterized by greenhouse period (mid-Cretaceous, Eocene, Miocene) followed by a
step-wise decrease in bottom-water temperatures and/or increased ice volume. Even almost
similar in shape and magnitide, however, subsequent cycles are represented by
cooler boundary conditions and therefore less pronounced greenhouse conditions. |
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