AAPG ACE 2018

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Leeuwin Current Activity Controls Benthic Foraminifera on the Carnarvon Ramp, Western Australia: From Inception to Flow Optimum

Abstract

The vast Mesozoic hydrocarbon reservoirs along the roughly 3000-km Western Australian Shelf (e.g., offshore Barrow Island) have a well-defined biostratigraphy. However, little is known about the overlying late Neogene and Holocene deposits, especially the timing and inception of the warm and oligotrophic Leeuwin Current (LC). The International Ocean Discovery Program Expedition 356 (July-Sept. 2015) recovered sediment core U1460A (Total Depth 300.1 m) from the Carnarvon Ramp (northern Perth Basin) in 215 m water depth and ca. 150 m adjacent to the commercially drilled well Morangie 1. Post-cruise, we analyzed 35 core catcher samples at ~9 m intervals for benthic foraminifera and associated size-fraction faunas to constrain biological evolution as a proxy for physical oceanographic changes.

Core catcher samples yielded predominantly well-preserved specimens. Cluster analysis and Non-Metric Multidimensional Scaling results identify three distinct biofacies, with an abrupt transition between 1 and 0.9 Ma. Foraminiferal Assemblage 1, dominated by shallow to deep infaunal taxa (Trifarina, Uvigerina, and Lagena), occurs from late Pliocene to mid Pleistocene (300–143 m; 3.54–0.91 Ma. These taxa are commonly related to high carbon flux, which may be derived from persistent deep-water upwelling from the cold Western Australian Current. A subtle Transitional Assemblage of epifaunal rotalid increase between 1.14–0.61 Ma marks the onset of the LC and its undercurrent, indicating increasing oxygen concentrations at the sediment-water interface. Foraminiferal Assemblage 2 was found from mid Pleistocene to present (133–0 m; 0.91–0 Ma), representing a more evenly distributed fauna and oligotrophic conditions. Species common at shallower depths (miliolids, agglutinated, and epiphytic forms) and epifaunal rotalids became more abundant as diversity increased. High sponge spicule concentration and sedimentation rate around 0.6 Ma signal a warm-water flux optimum originating from the Indonesian Throughflow. However, temporary higher abundance of opportunistic species such as Bolivina robusta indicates intensifying glacial cycles, which led to LC instability or lateral shift.

We conclude that the LC had its inception in the mid Pleistocene (1–0.91 Ma) and effectively reduced upwelling of the deep-water Western Australian Current. This timing is in concordance with emerging reef structures at 1 Ma on the Western Australian Shelf reported from seismic studies.