Understanding Bulk Sediment Stable Isotope Records in the Eastern Equatorial Pacific, From Seven Million Years Ago to Present Day

Abstract

Stable isotope (δ13C and δ18O) records of bulk marine sediment carry information on past carbon cycling and oceanography, but origins and interpretations remain uncertain because such signals represent mixtures of different biogenic components, each with potential offsets from primary parameters. Studies of Neogene sediment from the eastern equatorial Pacific (EEP) exemplify this issue, because stable isotope records of bulk sediment and foraminifera at different sites exhibit similarities and differences in absolute value that somehow relate to depositional age. Here we measure δ13C and δ18O of bulk carbonate, two fine‐grained fractions (<63 and <20 μm), mixed‐species planktic and benthic foraminifera, and foraminifera fragments from sediments deposited over four time intervals within the last 7 Ma at ODP Site 851. These data are compared to published δ13C and δ18O records of multiple single‐species planktic foraminifera from the same site and benthic foraminifera from an adjacent site. Bulk sediment δ13C and δ18O records represent a mixed signal dominated by reticulofenestrid coccolith calcite but modified by variable amounts of different foraminifera. Similarities and differences between stable isotope records result from temporal changes in water chemistry and temperature, depths of calcite precipitation, and vital effects that impact fractionation of various biogenic components. The remarkable correlation of bulk stable isotope records within the EEP suggests that several factors change collectively over time across a broad oceanographic region. Ideally, multiple stable isotope records coupled with other proxy measurements might lead to an internally consistent paleoceanographic perspective of the EEP since the late Miocene.

Size‐Fraction‐Specific Stable Isotope Variations as a Framework for Interpreting Early Eocene Bulk Sediment Carbon Isotope Records

“…Bulk carbonate δ 13 C values are similar to FF δ 13 C values throughout the late Paleocene‐early Eocene section at Shatsky Rise (Figure 3), primarily because the FF is the predominant component of bulk carbonate. This observation is similar to that found in the Neogene sediments from the eastern equatorial Pacific (Reghellin et al., 2020) and Paleogene sediments of the Cicogna section (Agnini et al., 2016). The fine fraction, dominated by calcareous nannofossils, tends to record conditions in the upper water column (e.g., Ennyu et al., ; Okada and Honjo, 1973), so bulk carbonate δ 13 C values in the early Eocene interval at Shatsky Rise likely reflect the conditions and δ 13 C values of inorganic carbon in surface waters.…”

“…While independent constraints on these processes are limited, temporal variations in the different size fractions of bulk carbonates, as well as their δ 13 C values, may provide insight on what drives both short‐ and long‐term variability in the carbon isotope record. The two different components in bulk carbonates―calcareous nannofossils (<20 μm) and foraminifers (>63 μm) ―can readily be segregated into different size fractions (Broecker & Clark, 1999, 2009; Chiu & Broecker, 2008; Reghellin et al., 2015; 2020). Changes in the relative abundance of these calcifying groups will certainly affect bulk isotope records and, consequently, how major paleoceanographic and paleoclimatic changes are expressed in the marine sedimentary record.…”

“…This difference arises because bulk carbonates at Site 1209 are dominated by calcareous nannofossils (Bralower et al., 2002), which generally do not exceed 20 µm in size (Broecker & Clark, 1999, 2009; Henderiks & Rickaby, 2007; O'Dea et al., 2014). Calcareous nannoplankton and benthic foraminifers reflect surface water and deep‐water signals, respectively, and the former usually has higher δ 13 C values than bottom waters (Figure 1; Bhattacharya and Dickens, 2020; Reghellin et al., 2020). Benthic foraminifers represent bottom‐water conditions, and thus have lower δ 13 C values than bulk carbonates (Figure 2).…”

“…These organisms can be readily grouped as either pelagic or benthic. Calcareous nannoplankton dominate among pelagic calcifiers, followed by planktic foraminifers (e.g., Bralower et al., 2002; Reghellin et al., 2020). Benthic foraminifers, by contrast, generally constitute a small proportion of open marine carbonates (Cramer et al., 2009; Milliman, 1993; Sarmiento & Gruber, 2006).…”

Climate, cryosphere and carbon cycle controls on Southeast Atlantic orbital-scale carbonate deposition since the Oligocene (30–0 Ma)

“…Conversely, there is no major late Miocene increase in benthic δ 18 O records relative to those seen mid Miocene and Pleistocene, indicating that there was no large expansion in continental ice sheet extent or substantial deep-sea cooling (Hodell et al, 2001;Drury et al, 2016Drury et al, , 2018bHolbourn et al, 2018;Tian et al, 2018). The carbon cycle underwent major change in the atmospheric, terrestrial and marine realms, with evidence for an atmospheric pCO2 decrease around ~8-7 Ma (Bolton and Stoll, 2013;Herbert et al, 2016;Mejía et al, 2017), the rise of terrestrial C4 plants on land (Cerling et al, 1997;Behrensmeyer et al, 2007;Uno et al, 2016;Tauxe and Feakins, 2020), and the globally synchronous marine late Miocene carbon isotope shift (LMCIS; ~7.5-6.9 Ma) linked to global changes in oceanic circulation (Haq et al, 1980;Hodell and Venz-Curtis, 2006;Reghellin et al, 2015Reghellin et al, , 2020Drury et al, 2017Drury et al, , 2018a.…”