Early Eocene climatic optimum: Environmental impact on the North Iberian continental margin

Abstract

The early Eocene climatic optimum, which constituted the peak of the long-term early Cenozoic global warming, had a significant impact on the environmental evolution of terrestrial and oceanic areas. Surprisingly, however, its influence on continental margins is poorly known. New insights are provided from a sedimentological, stable isotope, mineralogical, and micropaleontological study of an 1100-m-thick Lower-Middle Eocene deep-marine succession that accumulated on the North Iberian continental margin. The early Eocene climatic optimum is represented by a 410-m-thick interval characterized by scarcity of hemipelagic limestones, abundance of dark marls, which record a reduction in calcium carbonate content and an increase in kaolinite, and the occurrence of conspicuous red layers with high siderite and pyrite content.Series of stratigraphically significant events frame the early Eocene climatic optimum. Based on this analysis, the environmental influence of the early Eocene climatic optimum started at 52.6 Ma and lasted ~2.3 m.y. Its onset is marked by rapid drops in δ 13 C and δ 18 O, which record the addition of 13 C-depleted carbon into the ocean-atmosphere system for 80 k.y. and a concomitant warming. A hotter climate and a perennial rainfall regime increased the supply of terrestrial clays, organic matter, and iron oxides into the sea. Eventually, these changes affected the deepsea bottom 270 k.y. after the onset of the early Eocene climatic optimum, creating conditions in which opportunistic benthic foraminifera thrived, and leading to increased methanogenesis in the subsurface, which caused the formation of siderite. A subsequent gradual recovery culminated abruptly at 50.3 Ma with a global cooling episode, which is locally recorded by the accumulation of lowstand resedimentation deposits.

Stable isotope and calcareous nannofossil assemblage record of the late Paleocene and early Eocene (Cicogna section)

“…Like previous workers, we cannot discount the notion that temperatures at low and high latitudes responded differently across the early Paleogene (Pearson et al, 2007;Huber and Caballero, 2011). Unlike for carbon isotopes, however, local dissolution and re-precipitation of carbonate should significantly impact the δ 18 O of marine carbonate.…”

“…Indeed, the latter date marks the acme of the Early Eocene Climatic Optimum (EECO), the warmest sustained time interval of the Cenozoic (Zachos et al, 2008;Cramer et al, 2009;Hollis et al, 2012). Such a rise in temperature is not obvious at low latitudes with current data (Pearson et al, 2007;Huber and Caballero, 2011).…”

“…Samples were examined under a Zeiss light microscope at 1250× magnification. Calcareous nannofossils were determined using taxonomy proposed by Aubry (1984Aubry ( , 1988Aubry ( , 1989Aubry ( , 1990Aubry ( , 1999b, Perch-Nielsen (1985) and Bown (2005). , and DSDP Site 577 Dickens and Backman, 2013).…”

“…This is not yet established. For example, several recently published δ 13 C records (Kirtland-Turner et al, 2014;Slotnick et al, 2015a;Payros et al, 2015) do not precisely correlate with those at other locations Zachos et al, 2010;Slotnick et al, 2012Slotnick et al, , 2015b, at least with available stratigraphy. (2) How did marine calcifying organisms respond to major early Paleogene perturbations in temperature and carbon cycling, both in terms of evolution and preservation?…”

“…To the right is the general lithologic column and magnetostratigraphy of the Cicogna section (Dallanave et al, 2009). While numerous studies have examined calcareous nannofossils across the PETM from different perspectives (e.g., Bralower, 2002;Stoll and Bains, 2003;Gibbs et al, 2006a, b;Agnini et al, 2007a;Mutterlose et al, 2007;Bown and Pearson, 2009;Jiang and Wise, 2009;Self-Trail et al, 2012), the relationship between these organisms and carbon cycle perturbations before and after this short-lived warming episode remain poorly documented . It seems possible that the high rate of calcareous nannofossil taxonomic evolution (appearances and extinctions), as well as distinct changes in calcareous nannofossil abundance patterns, may provide excellent stratigraphic control across the early Paleogene (Bukry, 1973;Perch-Nielsen, 1985;Backman, 1986;Agnini et al, 2014). Moreover, if the exact relationship between changes in nannofossil assemblages and global carbon cycle perturbations were known, key time intervals could be rapidly identified for more detailed work.…”