 |
| Titel |
Modelling the Middle Miocene fluctuations in ocean biological productivity, ocean carbon isotopic composition and atmospheric CO2 level |
| VerfasserIn |
Ingrid Jacquemin, Louis Francois, Liselotte Diester-Haass, Katharina Billups, Kay-Christian Emeis |
| Konferenz |
EGU General Assembly 2011
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250056268
|
|
|
|
|
|
| Zusammenfassung |
| The Middle Miocene (17.5-13.5 Ma) is characterized by high frequency fluctuations in the
seawater δ13C record with six distinct δ13C maxima linked to Earth orbital cycles
(mainly the 400 kyr eccentricity cycle). These fluctuations are negatively correlated
with the δ18O variations and, on the other hand, marine data show that the ocean
productivity tends to be higher during warmer periods. However, on the long term,
these three indicators show a different relationship: the productivity does not show
any major trend, while a positive excursion of the δ13C is observed (the so-called
Monterey event) and is accompanied by a warming, i.e., a decrease in the δ18O. The
question of the long-term trend has already been addressed in a previous study
(Diester-Haass et al., Paleoceanography, 24, PA1209, 2008), where the positive
excursion in δ13C was explained by the storage in sediments of organic carbon from
continental origin. In the current study we hypothesize that the short-term (400 kyr cycle)
fluctuations of the marine δ13C are mainly linked to sea level and marine productivity
changes and not to changes in the terrestrial environment, explaining the different
relationship between the isotopic indicators for this shorter as compared to the
longer timescale. We test this hypothesis by using a global geochemical box model
representing the carbon, alkalinity, phosphorus and oxygen cycles and coupled to
an energy balance climate model. The model is forced with sea level fluctuations
associated with the observed variations of ocean δ18O. We first evaluate the changes in
continental weathering associated with these sea level fluctuations and the related
climate change. The model shows that these changes in weathering cannot explain
the observed amplitude of marine productivity and ocean δ13C variations over the
400 kyr eccentricity cycle. We then test the assumption that these high frequency
δ13C variations were the result of a change in the turnover rate of the oceans, with
higher upwelling rates and higher productivity during warmer periods. Results
of model sensitivity tests indicate that this hypothesis is plausible, since it leads
to approximately correct amplitudes and phases of the oceanic productivity and
δ13C signals. Thus, the following picture emerges that, in the Middle Miocene,
the short-term fluctuations in oceanic δ13C are driven by ocean circulation and
productivity changes, while the trends at longer timescales are linked to a change
in the burial rate of continental organic carbon. From this picture, the model is
able to provide a geochemical history of the Middle Miocene ocean-atmosphere
system consistent with the marine δ13C record. In this reconstructed history, the
atmospheric CO2 level only shows small variations, with a slight decrease from the Lower
Miocene (about 310 ppmv at 19 Ma) to the Middle Miocene (280 ppmv at 16 Ma)
followed by a small increase thereafter (310 to 320 ppmv near 12 Ma). This history is
relatively consistent with the marine 13C isotopic proxies for atmospheric CO2. |
|
|
|
|
|
|