3 research outputs found
Amazon forest dynamics under changing abiotic conditions in the early Miocene (Colombian Amazonia)
Amazon forest dynamics under changing abiotic conditions in the early Miocene (Colombian Amazonia)
A 26 million year gap in the central Arctic record at the greenhouse-icehouse transition: Looking for clues
The Cenozoic record of the Lomonosov Ridge (central Arctic Ocean) recovered during Integrated Ocean
Drilling Program (IODP) Expedition 302 revealed an unexpected 26 Ma hiatus, separating middle Eocene
(�44.4 Ma) from lower Miocene sediments (�18.2 Ma). To elucidate the nature of this unconformity, we
performed a multiproxy palynological (dinoflagellate cysts, pollen, and spores), micropaleontological
(siliceous microfossils), inorganic, and organic (Tetra Ether Index of lipids with 86 carbon atoms (TEX86)
and Branched and Isoprenoid Tetraether (BIT)) geochemical analysis of the sediments from �5 m below to
�7 m above the hiatus. Four main paleoenvironmental phases (A–D) are recognized in the sediments
encompassing the unconformity, two below (A–B) and two above (C–D): (A) Below the hiatus, proxies show
relatively warm temperatures, with Sea Surface Temperatures (TEX86-derived SSTs) of about 8�C and high
fresh to brackish water influence. (B) Approaching the hiatus, proxies indicate a cooling trend (TEX86-derived
SSTs of �5�C), increased freshwater influence, and progressive shoaling of the Lomonosov Ridge drilling
site, located close to or at sea level. (C) The interval directly above the unconformity contains sparse reworked
Cretaceous to Oligocene dinoflagellate cysts. Sediments were deposited in a relatively shallow, restricted
marine environment. Proxies show the simultaneous influence of both fresh and marine waters, with
alternating oxic and anoxic conditions. Pollen indicates a relatively cold climate. Intriguingly, TEX86-derived
SSTs are unexpectedly high, �15–19�C. Such warm surface waters may be partially explained by the
ingression of warmer North Atlantic waters after the opening of the Fram Strait during the early Miocene. (D)
Sediments of the uppermost interval indicate a phase of extreme oxic conditions, and a well-ventilated
environment, which occurred after the complete opening of the Fram Strait. Importantly, and in contrast with
classical postrifting thermal subsidence models for passive margins, our data suggest that sediment erosion
and/or nondeposition that generated the hiatus was likely due to a progressive shoaling of the Lomonosov
Ridge. A shallow water setting both before and after the hiatus suggests that the Lomonosov Ridge remained
at or near sea level for the duration of the gap in the sedimentary record. Interacting sea level changes and/or
tectonic activity (possibly uplift) must be invoked as possible causes for such a long hiatus