High resolution cyclostratigraphy of the early Eocene – new insights into the origin of the Cenozoic cooling trend
Other literature type
(issn: 1814-9332, eissn: 1814-9332)
Here we present a high-resolution cyclostratigraphy based on X-ray
fluorescence (XRF) core scanning data from a new record retrieved from the
tropical western Atlantic (Demerara Rise, ODP Leg 207, Site 1258). The
Eocene sediments from ODP Site 1258 cover magnetochrons C20 to C24 and show
well developed cycles. This record includes the missing interval for
reevaluating the early Eocene part of the Geomagnetic Polarity Time Scale
(GPTS), also providing key aspects for reconstructing high-resolution
climate variability during the Early Eocene Climatic Optimum (EECO).
Detailed spectral analysis demonstrates that early Eocene sedimentary cycles
are characterized by precession frequencies modulated by short (100 kyr) and
long (405 kyr) eccentricity with a generally minor obliquity component.
Counting of both the precession and eccentricity cycles results in revised
estimates for the duration of magnetochrons C21r through C24n. Our
cyclostratigraphic framework also corroborates that the geochronology of the
Eocene Green River Formation (Wyoming, USA) is still questionable mainly due
to the uncertain correlation of the "Sixth tuff" to the GPTS.
Right at the onset of the long-term Cenozoic cooling trend the dominant
eccentricity-modulated precession cycles of ODP Site 1258 are interrupted by
strong obliquity cycles for a period of ~800 kyr in the middle of
magnetochron C22r. These distinct obliquity cycles at this low latitude site
point to (1) a high-latitude driving mechanism on global climate variability
from 50.1 to 49.4 Ma, and (2) seem to coincide with a significant drop in
atmospheric CO<sub>2</sub> concentration below a critical threshold between 2- and
3-times the pre-industrial level (PAL). The here newly identified orbital
configuration of low eccentricity in combination with high obliquity
amplitudes during this ~800-kyr period and the crossing of a critical
<i>p</i>CO<sub>2</sub> threshold may have led to the formation of the first ephemeral ice
sheet on Antarctica as early as ~50 Ma ago.