Temperature Gradients Across the Pacific Ocean During the Middle Miocene

Abstract

Sea surface temperatures (SSTs) of the tropical Pacific Ocean exert powerful controls on regional and global climates. Previous studies have suggested that during warm climate phases, the east‐west temperature gradient collapsed. To date, there has been no high‐resolution reconstruction of sea surface conditions in both the east and west Pacific Ocean during the Miocene Climate Optimum (MCO) and across the middle Miocene climate transition (MMCT); therefore, our understanding of the mean oceanographic state during this major global climatic shift is limited. Here, we present new SST reconstructions for the eastern Pacific Ocean (15.5–13.3 Ma) which show a clear east‐west temperature gradient of ∼4°C during the warmest interval of the Neogene, implying that the oceanographic processes that produce the modern gradient were present and active. There is no shift in the east‐west gradient across the MMCT indicating that the gradient was not impacted by global cooling and ice growth. We find a 2°C sea surface cooling in the eastern equatorial Pacific, that lags the benthic foraminiferal δ18O positive shift by 150 kyr, indicating that tropical temperature did not decrease synchronously with the expansion of the Antarctic ice sheet. Reconstructed variations in the δ18O composition of seawater, determined by combining our Mg/Ca and δ18O records, reveal a freshening in the eastern Pacific Ocean after 13.8 Ma, suggesting changes in the hydrological cycle and in tropical fronts in response to the new icehouse regime.

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

“…Dinoflagellates are unicellular, eukaryotic, mostly planktonic protists that occur in all aquatic environments but with highest diversity in the marine realm (Fensome, 1993). Dinoflagellates have several trophic strategies and can be heterotrophic, mixotrophic and autotrophic.…”

Atlantic Meridional Overturning Circulation Influence on the Annual Mean Intertropical Convergence Zone Location in the Miocene

“…Pink and blue filled circles indicate the meridional location of the maximum precipitation in the tropics at each longitude from AMOC‐off and AMOC‐on , respectively. Crossed circles represent the sites where proxy‐derived ocean surface freshening was found for the middle Miocene (Fox et al., 2021; Holbourn et al., 2010; Yang et al., 2020). The gray color indicates areas without a statistically significant difference at the 95% confidence interval, calculated using a two‐tailed Student's t ‐test.…”

“…While direct precipitation records indicating the oceanic ITCZ shift in the early‐to‐middle Miocene are scarce, estimates of ocean mixed layer δ 18 O from geochemical records suggest freshening at one site in the tropical Indian Ocean (Yang et al., 2020) and two sites in the tropical Pacific (Fox et al., 2021; Holbourn et al., 2010) during the middle Miocene (Figure 2). This surface freshening has been attributed to the northward shift of the ITCZ (Fox et al., 2021; Holbourn et al., 2010; Yang et al., 2020). Our results generally agree with these proxy studies, but additional proxy records for the oceanic ITCZ location and its shifts are needed to further validate our simulations.…”

Paleoceanographic Implications of Diatom Seasonal Laminations in the Upper Miocene Pisco Formation (Ica Desert, Peru) and Their Clues on the Development of the Pisco Fossil‐Lagerstätte

“…Although the discussion on the existence of an El Padre State lies outside the objectives of this paper, we would like to point out that: (a) there is no doubt that the CLQ20 Coscinodiscus laminae point to a deepening of the thermocline in the Messinian and it is stunning to observe that the deposition of CLQ20 happened during a period when SST in the EEP were increasing (Figure 3) and (b) a primary production segregated at the thermocline depth may help explain the so-called "Pliocene paradox" (Shankle et al, 2021), which highlights enhanced primary production in the EEP despite a reduced latitudinal gradient (which, as said, would imply a deepening of the thermocline in the EEP) in the Late Miocene/Early Pliocene; and (c) the BSE images of CLQ20 highlights that shade flora can represent a huge percentage of the total diatom assemblage; therefore, given the importance of the carbon export attributable to the thriving of the shade flora at depth, the use of biogenic silica as a proxy for intensification of upwelling, as done in some researches (e.g., Fox et al, 2021;Holbourn et al, 2014. Also Esperante et al, 2015 suggest that the abundant occurrence of Thalassionema nitzschioides in the sediment of the Pisco Fm.…”

“…The hypothesis on whether the El Padre state either existed or is just a result of a bias in the proxies (TEX 86 , U K’ 37 , Mg/Ca) used for the reconstruction of the SST gradient is still ongoing (Ravelo et al., 2014; White and Ravelo 2020b; Zhang et al., 2014a, 2014b, and reference therein), also extending into the Middle Miocene (Fox et al., 2021).…”