The Middle to Late Miocene “Carbonate Crash” in the Equatorial Indian Ocean

Julia Lübbers,Emmeline Gray,Karlos G D Kochhann,Wolfgang Kuhnt,Sebastian Beil,Nils Andersen,Clara T Bolton,Ann E Holbourn,Yoichi Usui

doi:10.1029/2018pa003482

Abstract

AbstractWe integrate benthic foraminiferal stable isotopes, X‐ray fluorescence elemental ratios, and carbonate accumulation estimates in a continuous sedimentary archive recovered at International Ocean Discovery Program Site U1443 (Ninetyeast Ridge, Indian Ocean) to reconstruct changes in carbonate deposition and climate evolution over the interval 13.5 to 8.2 million years ago. Declining carbonate percentages together with a marked decrease in carbonate accumulation rates after ~13.2 Ma signal the onset of a prolonged episode of reduced carbonate deposition. This extended phase, which lasted until ~8.7 Ma, coincides with the middle to late Miocene carbonate crash, originally identified in the eastern equatorial Pacific Ocean and the Caribbean Sea. Interocean comparison reveals that intense carbonate impoverishment at Site U1443 (~11.5 to ~10 Ma) coincides with prolonged episodes of reduced carbonate deposition in all major tropical ocean basins. This implies that global changes in the intensity of chemical weathering and riverine input of calcium and carbonate ions into the ocean reservoir were instrumental in driving the carbonate crash. An increase in U1443 Log (Ba/Ti) together with a change in sediment color from red to green indicate a rise in organic export flux to the sea floor after ~11.2 Ma, which predates the global onset of the biogenic bloom. This early rise in export flux from biological production may have been linked to increased advection of nutrients and intensification of upper ocean mixing, associated with changes in the seasonality and intensity of the Indian Monsoon.

Highlights

During the middle to late Miocene, Earth's climate transitioned from a warmer phase with a reduced and highly dynamic Antarctic ice sheet to a colder, more permanently glaciated mode (Figure 1)
We used the δ18O minimum at the onset of the peak warm event at 10.7 Ma as a primary tie point, supplemented by four δ13C, two δ18O minima and two δ13C maxima, which are distinctive in both records
Based on the carbonate content (% and mass accumulation rates (MARs)), we identify three distinct stages in the evolution of carbonate deposition at Site U1443 (Figure 5). (1) A slight declining trend in carbonate content together with a distinct decrease in MARs starting at ~13.2 Ma mark the initial phase, which lasted until ~12.2 Ma

Summary

Introduction

During the middle to late Miocene, Earth's climate transitioned from a warmer phase with a reduced and highly dynamic Antarctic ice sheet to a colder, more permanently glaciated mode (Figure 1). This extended interval provides insights into different modes of natural climate variability on a warmer Earth, predominantly influenced by a Southern Hemisphere ice sheet. Our understanding of underlying processes driving climate change following pulses of glacial expansion and global cooling at ~13.9 and ~13.1 Ma remains enigmatic, because sediment successions within the interval 13 to 8 Ma are often incomplete due to pervasive carbonate dissolution. Whereas most studies of the carbonate crash have focused on the eastern equatorial Pacific Ocean, its impact on oligotrophic regions of the global ocean remains contentious (e.g., Berger et al, 1993; Diester‐Haass et al, 2004; Nathan & Leckie, 2009; Preiß‐ Daimler et al, 2013)

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Results

Discussion

Conclusion