Early to Middle Miocene Astronomically Paced Climate Dynamics in the Eastern Equatorial Atlantic

Abstract

Detailed analysis of tropical climate dynamics is lacking for the Early to Middle Miocene, even though this time interval bears important analogies for future climates. Based on high‐resolution proxy reconstructions of sea surface temperature, export productivity and dust supply at Ocean Drilling Program Site 959, we investigate astronomical forcing of the West African monsoon in the eastern equatorial Atlantic across the prelude, onset, and continuation of the Miocene Climatic Optimum (MCO; 18–15 Ma). Along with previously identified eccentricity periodicities of ∼400 and ∼100 kyr, our records show that climate varied on ∼27–17 kyr, ∼41 kyr, and ∼60–50 kyr timescales, which we attribute to precession, obliquity, and their combination tones, respectively. The relative contribution of these astronomical cycles differed between proxies and through time. Three intervals with distinct variability were recognized, which are particularly clear in the temperature record: (a) strong eccentricity, obliquity, and precession variability prior to the MCO (18.2–17.7 Ma), (b) strong influence of obliquity just after the onset of the MCO (16.9–16.3 Ma) concurring with a 2.4 Myr eccentricity minimum, and (c) dominant eccentricity and precession variability during the MCO between 16.3 and 15.0 Ma. Sedimentation at Site 959 was influenced by astronomically paced variations in upwelling intensity and North African aridity related to West African monsoon dynamics. Continuously present patterns of precession imply low‐latitude forcing, while asymmetric eccentricity and obliquity imprints and strong obliquity influence suggest that Site 959 was also affected by high‐latitude, glacial‐interglacial dynamics.

Tropical Warming and Intensification of the West African Monsoon During the Miocene Climatic Optimum

“…As this increased variability in our SST, export productivity and dust records are unmatched in the younger parts of the MCO (∼16.5-15.0 Ma) as well as prior to the MCO onset (∼18.2-17.7 Ma), we conclude that WAM circulation was intensified immediately following the onset of the MCO. The strong orbital control on the WAM intensity variations (Spiering et al, 2024) corroborates this conclusion.…”

“…Prior to the MCO, SST variability of ±4°C follows 100 kyr eccentricity cycles (Spiering et al, 2024). Two 100 kyr eccentricity SST peaks between ∼17.9-17.7 Ma show a remarkable asymmetric shape which could point to an influence of high-latitude glacial variability (Spiering et al, 2024). Following the onset of the MCO (∼16.9-16.5 Ma), SST variability remains remarkably high and even increases to on average ±5°C.…”

“…In this case, alongshore westward flow causes offshore Ekman transport of surface waters, thereby creating divergence to facilitate upwelling (Verstraete, 1992). Therefore, when the ITCZ was displaced most southernly during precession maxima, upwelling would have intensified, which caused elevated export productivity from surface waters at Site 959 (Spiering et al, 2024). As was proposed for the Late Miocene to Pleistocene (van der Weijst et al, 2022), and as we surmise for the pre-MCO interval of our records, upwelled waters were likely sourced from SACW, which is approximately 10-20°C colder than the surface waters in the GG.…”

“…The SST record at Site 959 (n = 554) shows a warming of ∼1.5°C, based on the TEX 86 H calibration, from the earlier Miocene (∼18.3-17.7 Ma; average SST 28.4°C) to the MCO (<16.9 Ma; average SST 29.9°C, Figure 3a). Prior to the MCO, SST variability of ±4°C follows 100 kyr eccentricity cycles (Spiering et al, 2024). Two 100 kyr eccentricity SST peaks between ∼17.9-17.7 Ma show a remarkable asymmetric shape which could point to an influence of high-latitude glacial variability (Spiering et al, 2024).…”

“…Here, we present TEX 86 (TetraEther Index of tetraethers with 86 carbon atoms; Schouten et al, 2002Schouten et al, , 2013 based seawater temperatures and organic dinoflagellate cyst (dinocyst) assemblages across the onset of the MCO at Site 959 (∼18.3-15.0 Ma) and combine these records with previously published lithological and geochemical data from the same samples (Spiering et al, 2024;Wubben et al, 2023). Our high-resolution sampling provides an unprecedented evaluation of surface water conditions during the MCO and allows us to assess (a) warming in the tropics during the MCO, and (b) the effect of tropical warming on WAM circulation by tracing variability in processes such as (export) productivity, upwelling and dust supply on precession to eccentricity timescales.…”