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

Abstract

Studying monsoon dynamics during past warm time periods such as the Miocene Climatic Optimum (MCO; ∼16.9–14.5 Ma) could greatly aid in better projecting monsoon intensity, in the context of future greenhouse warming. However, studies on regional MCO temperature change and its effect on the monsoons during this time period are lacking. Here, we present the first high‐resolution, low‐latitude record of sea surface temperature (SST) and paleoceanographic change covering the Miocene Climatic Optimum, in the eastern equatorial Atlantic, at Ocean Drilling Program Site 959, based on TEX86 paleothermometry. SSTs were ∼1.5°C warmer at the onset of the MCO (16.9 Ma) relative to the pre‐MCO (∼18.3–17.7 Ma). This warming was accompanied by a transient increase in %total organic carbon. Prior to the MCO, sediment composition, geochemical proxy data as well as dinoflagellate cyst assemblages imply a productive surface ocean at Site 959. Immediately following the MCO onset (∼16.9–16.5 Ma), we record an intensification of the West African Monsoon (WAM) characterized by higher amplitude variability in all proxy records on precession to obliquity timescales. We interpret increased orbital‐scale SST, biogenic Ba and dinocyst assemblage variability to represent intensification of equatorial upwelling, forced by the WAM strength. Furthermore, higher SSTs during eccentricity maxima correlate to increased relative abundances of the warm and stratification‐favoring dinocyst Polysphaeridium zoharyi, during periods of low WAM intensity. Finally, while long‐term SSTs decline toward the middle Miocene, maximum SSTs and Polysphaeridium zoharyi abundances occur during MCO peak warming at ∼15.6 Ma.

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

“…The Early to Middle Miocene proxy records of Site 959 reveal periodicities of ∼400 and ∼100 kyr, ∼41 kyr, and ∼27-17 kyr, which are likely related to ∼400 and ∼100 kyr eccentricity, obliquity, and precession, respectively, and additionally ∼60-50 kyr cyclicity. Fluctuations of TEX 86 reflect temperature variations of the shallow subsurface and were interpreted to represent variations in upwelling intensity, consistent with dinoflagellate cyst analyses (Wubben et al, 2024). As upwelling boosts productivity, this is in agreement with the occurrence of productivity (Ba bio ) peaks during SST (TEX 86 ) minima.…”

“…Alternatively, the degree of stratification was dependent on the amplitude of precession, resulting in greater stratification and higher temperatures during eccentricity maxima, as observed. This agrees with the increased abundance of dinoflagellate cyst Polysphaeridium during TEX 86 maxima, which indicates extreme stratification of surface waters (hyperstratification; Wubben et al, 2024). The temperature sensitivity to one side of the precession cycle indicates that the response was highly non-linear (Figure 11).…”

“…Peaks of Ti/Al are interpreted as periods of increased dust supply and occurred when upwelling was intensified. Terrestrial input was mainly aeolian, as Site 959 was not strongly affected by fluvial processes during the Early to Middle Miocene (Wubben et al, 2024). These cyclic variations in upwelling intensity and dust supply point to changes in atmospheric and hydrologic circulation related to the presence of a monsoon system.…”

“…The sediment color, magnetic susceptibility (MS), bulk carbonate stable carbon and oxygen isotopic ratios (δ 13 C and δ 18 O) and wt% CaCO 3 records of Wubben et al (2023) have an average resolution of 1 cm (<1 kyr), 3 cm (1 kyr), 5 cm (2 kyr) and 15 cm (5 kyr), respectively (Figure 4). A record of the tetraether index of tetraethers with 86 carbon atoms (TEX 86 ;Schouten et al, 2002), which is positively correlated to SST, was generated by Wubben et al (2024) at an average resolution of 13 cm (4 kyr). We also consider their record of the branched and isoprenoid tetraether (BIT) index (Hopmans et al, 2004) for soil-derived biomarker inputs.…”

“…A recent age model for the Lower to Middle Miocene sediments showed that a near-complete 18-15 Ma section was recovered, suitable for assessing climate variability at precession to eccentricity timescales (Wubben et al, 2023). We use records of sediment color, magnetic susceptibility (MS), bulk carbonate stable carbon and oxygen isotopic composition (δ 13 C and δ 18 O) and weight percent (wt%) CaCO 3 published in Wubben et al (2023), biomarker paleothermometer data from a companion paper (Wubben et al, 2024), as well as newly presented bulk sediment elemental composition data (biogenic Ba, Ti/Al, and V/Al). Detailed frequency analysis is performed on these high-resolution records to investigate the response to astronomical forcing across the prelude, onset, and continuation of the MCO.…”