Paleoenvironmental Changes at ODP Site 702 (South Atlantic): Anatomy of the Middle Eocene Climatic Optimum

Abstract

The Middle Eocene Climatic Optimum (MECO) was an unusual global warming event that interrupted the long-term Eocene cooling trend ca. 40 Ma. Here we present new high-resolution bulk and benthic isotope records from South Atlantic ODP Site 702 to characterize the MECO at a high latitude setting. The MECO event, including early and peak warming as well as recovery to background levels, had an estimated~300 Kyr duration (~40.51 to~40.21 Ma). Cross-plots (δ 18 O vs. δ 13 C) suggest that the mechanisms driving coupled changes in O and C isotope values across the MECO were weaker or absent before the event. The paleoecological response has been evaluated by quantitative analysis of calcareous nannofossils and benthic foraminifera assemblages. We document a shift in the biogeographical distribution of warm and temperate calcareous nannoplankton taxa, which migrated toward higher latitudes due to increased temperatures during the MECO. Conversely, changes in the organic matter flux to the seafloor appear to have controlled benthic foraminifera dynamics at Site 702. Benthic phytodetritus exploiting taxa increased in abundance coinciding with a positive δ 13 C excursion,~150 Kyr before the start of the δ 18 O negative excursion that marks the start of MECO warming. Our data suggest that paleoecological disturbance in the deep sea predates MECO δ 18 O excursion and that it was driven by changes in the type and/or amount of organic matter reaching the seafloor rather than by increased temperature.

New composite bio- and isotope stratigraphies spanning the Middle Eocene Climatic Optimum at tropical ODP Site 865 in the Pacific Ocean

“…Mita, 2001;Tori and Monechi, 2013;Larrasoaña et al, 2008;Backman, 1987). The relatively high position of the Base of R. (C.) reticulata at Site 865 is also consistent with the highly variable position of this event reported from other sites, ranging from magnetochron C20r to C18n.2n (a ∼ 5 Myr interval; Rivero-Cuesta et al, 2019;Fioroni et al, 2012). However, given the sporadic and rare presence of R. (C.) reticulata in our own observations here, the most likely explanation is ecological bias at Site 865.…”

“…XRF data are routinely employed for stratigraphic correlation and the construction of high-resolution composite depth scales because of their higher signal-to-noise ratio and more consistent hole-to-hole character than shipboard physical property measurements (Röhl and Abrams, 2000). Prior to XRF analysis, the top ∼ 0.5 cm of the split section surface of the archive halves was scraped off with a glass slide.…”

“…The most pronounced of these is the MECO event at ∼ 40 Ma that represents a temporary reversal in the long-term Eocene global cooling trend (Bohaty and Zachos, 2003;Bohaty et al, 2009). The MECO event is characterized by ocean warming of ∼ 3-6 • C (Bijl et al, 2010;Bohaty et al, 2009;Boscolo Galazzo et al, 2014;Cramwinckel et al, 2018;Rivero-Cuesta et al, 2019), ocean acidification evidenced by > 1 km shoaling of the Pacific calcite compensation depth (CCD), and shifts in the global carbon cycle including a rise in atmospheric carbon dioxide concentrations (Bohaty et al, 2009;Pälike et al, 2012;Henehan et al, 2020;Bijl et al, 2010). Marine biotic responses vary across the event, impacting floral and faunal composition, body size, and ecology in the deep and surface ocean globally (e.g.…”

“…The MECO is well known from mid-and high-latitude sites (e.g. ODP Sites 689, 690, 702, 748, 738, 1051, 1172and 1263; Alum Bay in the UK; the Baskil section in Turkey; and the Alano, Contessa Highway, and Monte Cagnero sections in Italy), many of which also are characterized by good stratigraphic age control, comprehensive isotope stratigraphies, and carbonate-rich sediments yielding microfossils useful for proxy reconstructions Bohaty et al, 2009;Bohaty and Zachos, 2003;Bijl et al, 2010;Boscolo Galazzo et al, 2014;Spofforth et al, 2010;Jovane et al, 2007;Savian et al, 2013;Rivero-Cuesta et al, 2019;Giorgioni et al, 2019). However, we currently lack a low-latitude site that unambiguously spans the pre-MECO, MECO, and post-MECO intervals and has continuous carbonate sedimentation uninterrupted by CCD or lysocline shoaling, thus recording environmental and biotic responses in the tropics.…”

Searching for Function: Reconstructing Adaptive Niche Changes Using Geochemical and Morphological Data in Planktonic Foraminifera

“…The early to middle Eocene is punctuated by multiple short-lived (∼40-200 kyrs) transient global warming events or "hyperthermals" (Westerhold et al, 2020). During the middle Eocene there was a ∼270-500-kyr transient warming event known as the Middle Eocene Climatic Optimum that interrupted the long-term cooling trend (Bohaty and Zachos, 2003;Bohaty et al, 2009;Rivero-Cuesta et al, 2019;Edgar et al, 2020). The MECO was recognized by a progressive shift to lower deep-sea δ 18 O records between ∼40.6 and 40 Ma followed by an abrupt return to higher δ 18 O values (Bohaty et al, 2009).…”