Astrochronology of the Miocene Climatic Optimum record from Ocean Drilling Program Site 959 in the eastern equatorial Atlantic

The Miocene Climatic Optimum (MCO; ~16.9–14.7 Ma) was a relatively warm interval which interrupted the Cenozoic cooling trend and bears analogies with projected near-future climate change. Evidence for MCO warming and climatic variability is dominantly based on studies from mid- to high-latitude regions and deep ocean benthic foraminiferal oxygen isotope reconstructions, whereas studies from tropical latitudes are needed to resolve latitudinal temperature gradients and ocean nutrient cycling. Sedimentary cores retrieved at Ocean Drilling Program (ODP) Site 959 (Leg 159) in the eastern equatorial Atlantic Ocean offer a near-continuous, low-latitude record spanning the Early to Middle Miocene, but age constraints were limited. To achieve an orbitally resolved age model, we generated new calcareous nannofossil and diatom biostratigraphy as well as high-resolution bulk carbonate stable carbon (δ13C) and oxygen isotope (δ18O) ratios, magnetic susceptibility (MS), weight percent CaCO3 and mean greyscale records. We record several diagnostic biostratigraphic markers and identify the well-dated onset of the MCO, Monterey Excursion (ME), Carbon Maxima (CM) events and peak warming in the bulk carbonate isotope records. An orbital age model is realized by tuning the bulk carbonate δ13C record to eccentricity extracted from the Laskar et al. (2004) astronomical solution that is consistent with the bio- and chemostratigraphic constraints. We conclude that the studied sediment record spans the Early to Middle Miocene interval between ~18.2 and 15 Ma and includes a hiatus directly prior to the onset of MCO of maximally ~700 kyr. All records reveal dominant eccentricity-paced variability, while prominent precession and obliquity paced variability is observed between ~16.9–16.1 Ma; this interval corresponds to a node in the long ~2.4 Myr eccentricity cycle. Moreover, a major shift from bio-siliceous-dominated to carbonate-rich sediments is found across the onset of the MCO (~16.9 Ma). Both lithologies represent intervals of relatively high productivity, likely associated with upwelling. Ultimately, our high-resolution record from Site 959 can provide an important opportunity for reconstructing a tropical paleoclimate record at precession-to-eccentricity resolution during the MCO

A 15-million-year surface- and subsurface-integrated TEX86 temperature record from the eastern equatorial Atlantic

TEX86 is a paleothermometer based on Thaumarcheotal glycerol dialkyl glycerol tetraether (GDGT) lipids and is one of the most frequently used proxies for sea-surface temperature (SST) in warmer-than-present climates. However, GDGTs are not exclusively produced in and exported from the mixed layer, so sedimentary GDGTs may contain a depth-integrated signal that is also sensitive to local subsurface temperature variability. In addition, the correlation between TEX86 and SST is not significantly stronger than that to depth-integrated mixed-layer to subsurface temperatures. The calibration of TEX86 to SST is therefore controversial. Here we assess the influence of subsurface temperature variability on TEX86 using a downcore approach. We present a 15 Myr TEX86 record from Ocean Drilling Program Site 959 in the Gulf of Guinea and use additional proxies to elucidate the source of the recorded TEX86 variability. Relatively high GDGT[2/3] ratio values from 13.6 Ma indicate that sedimentary GDGTs were partly sourced from deeper (> 200 m) waters. Moreover, late Pliocene TEX86 variability is highly sensitive to glacial–interglacial cyclicity, as is also recorded by benthic δ18O, while the variability within dinoflagellate assemblages and surface/thermocline temperature records (Uk370 and Mg/Ca) is not primarily explained by glacial–interglacial cyclicity. Combined, these observations are best explained by TEX86 sensitivity to sub-thermocline temperature variability. We conclude that TEX86 represents a depth-integrated signal that incorporates a SST and a deeper component, which is compatible with the present-day depth distribution of Thaumarchaeota and with the GDGT[2/3] distribution in core tops. The depth-integrated TEX86 record can potentially be used to infer SST variability, because subsurface temperature variability is generally tightly linked to SST variability. Using a subsurface calibration with peak calibration weight between 100 and 350 m, we estimate that east equatorial Atlantic SST cooled by ∼ 5◦C between the Late Miocene and Pleistocene. On shorter timescales, we use the TEX86 record as a proxy for South Atlantic Central Water (SACW), which originates from surface waters in the South Atlantic Gyre and mixes at depth with Antarctic Intermediate Water (AAIW). Leads and lags around the Pliocene M2 glacial (∼ 3.3 Ma) in our record, combined with published information, suggest that the M2 glacial was marked by SACW cooling during an austral summer insolation minimum and that decreasing CO2 levels were a feedback, not the initiator, of glacial expansion

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

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

A 15-million-year surface- and subsurface-integrated TEX86 temperature record from the eastern equatorial Atlantic

TEX86 is a paleothermometer based on Thaumarcheotal glycerol dialkyl glycerol tetraether (GDGT) lipids and is one of the most frequently used proxies for sea-surface temperature (SST) in warmer-than-present climates. However, GDGTs are not exclusively produced in and exported from the mixed layer, so sedimentary GDGTs may contain a depth-integrated signal that is also sensitive to local subsurface temperature variability. In addition, the correlation between TEX86 and SST is not significantly stronger than that to depth-integrated mixed-layer to subsurface temperatures. The calibration of TEX86 to SST is therefore controversial. Here we assess the influence of subsurface temperature variability on TEX86 using a downcore approach. We present a 15 Myr TEX86 record from Ocean Drilling Program Site 959 in the Gulf of Guinea and use additional proxies to elucidate the source of the recorded TEX86 variability. Relatively high GDGT[2/3] ratio values from 13.6 Ma indicate that sedimentary GDGTs were partly sourced from deeper (> 200 m) waters. Moreover, late Pliocene TEX86 variability is highly sensitive to glacial–interglacial cyclicity, as is also recorded by benthic δ18O, while the variability within dinoflagellate assemblages and surface/thermocline temperature records (Uk370 and Mg/Ca) is not primarily explained by glacial–interglacial cyclicity. Combined, these observations are best explained by TEX86 sensitivity to sub-thermocline temperature variability. We conclude that TEX86 represents a depth-integrated signal that incorporates a SST and a deeper component, which is compatible with the present-day depth distribution of Thaumarchaeota and with the GDGT[2/3] distribution in core tops. The depth-integrated TEX86 record can potentially be used to infer SST variability, because subsurface temperature variability is generally tightly linked to SST variability. Using a subsurface calibration with peak calibration weight between 100 and 350 m, we estimate that east equatorial Atlantic SST cooled by ∼ 5◦C between the Late Miocene and Pleistocene. On shorter timescales, we use the TEX86 record as a proxy for South Atlantic Central Water (SACW), which originates from surface waters in the South Atlantic Gyre and mixes at depth with Antarctic Intermediate Water (AAIW). Leads and lags around the Pliocene M2 glacial (∼ 3.3 Ma) in our record, combined with published information, suggest that the M2 glacial was marked by SACW cooling during an austral summer insolation minimum and that decreasing CO2 levels were a feedback, not the initiator, of glacial expansion