Underlying causes for long-term global ocean δ13C fluctuations over the last 1.20 Myr
Introduction
Systematic oceanic carbon isotope (δ13C) shifts are commonly used indicators of change in the global carbon cycle, with research focussing on such shifts at the Cretaceous/Paleogene boundary [1], in the Miocene [2], [3], and at the Palaeocene–Eocene boundary [4]. The main interest of this study concerns sustained long-term δ13C shifts at timescales that exceed glacial/interglacial variability over the last 1.20 Myr, coincident with the possibly earliest onset of the Mid-Pleistocene climate transition [5]. In essence, the fluctuations considered here are background fluctuations that underlie the superimposed δ13C variability associated with the 41 to 100-kyr glacial cycles.
Two global long-term δ13C fluctuations can be identified during the last 1.20 Myr in records from both surface dwelling (planktonic) and bottom dwelling (benthic) foraminifera from all major ocean basins (Fig. 1, Supplementary information 1). Planktonic foraminiferal δ13C records reflect conditions in a thin surface layer with a high potential variability on regional scales while benthic foraminiferal δ13C records are more indicative of long-term stabilised properties in the oceanic deep-water reservoir that forms > 75% of the global ocean volume. Therefore we focus on the benthic foraminiferal δ13C records.
During the last 1.20 Myr, benthic foraminiferal δ13C records show similar long-term trends (i.e. trends exceeding variations on times scales up to and including glacial/interglacial time scales) (Fig. 2). The two long-term fluctuations recognized have been termed the Mid-Pleistocene δ13C Fluctuation (MPCF; 1.00–0.50 Ma) and the Late-Pleistocene δ13C Fluctuation (LPCF; 0.50–0.10 Ma). δ13C differences between the sites are related to water-mass ageing and mixing (Fig. 2). Today, the δ13C value of freshly formed North Atlantic Deep Water is ∼ 1‰, decreasing to ∼ 0‰ en route to the North Pacific, through mixing with Southern Ocean Water and continued remineralization of organic matter [11]. Despite these relative offsets between records from different basins, they all show the same systematic Pleistocene long-term δ13C variability, thus confirming the global nature of these fluctuations (Fig. 2).
Initial explanations of the ca. 0.35‰ drop in δ13C values at the start of the MPCF maintained that a drop in the mean ocean δ13C was due to a one-time net addition of 12C-enriched carbon to the world ocean [12]. This could have been caused by: (1) an increase in continental weathering and runoff; (2) an addition of organic matter to the ocean/atmosphere carbon reservoir from continental shelves; or (3) a global increase in aridity and decrease in the size of the biosphere across the 41 to 100-kyr transition [12]. The first two options were discarded as such processes would also enhance ocean nutrient concentrations and thus increase vertical δ13C gradients and the spatial δ13C gradient between the Atlantic and Pacific Oceans [13], whereas records suggest that these gradients remained relatively constant [12]. Raymo et al. [12] rule out a shift in the dominance of C3/C4 terrestrial ecosystems, as this would require a relatively large shift. In addition, the size of the terrestrial biosphere would have to decrease by more than 60% in order to explain the drop in δ13C values at the start of the MPCF, which seems unlikely [12].
The MPCF is approximately coincident with the mid-Pleistocene Climate Transition (MPT), which is marked by an increase in mean global ice volume and a shift from 41-kyr to 100-kyr ice-age cycles [12]. The timing of the initiation of the LPCF can be broadly linked to the mid-Brunhes event which has also been associated with an increase in global ice volume [14]. It has been suggested that the ice-sheet expansion at the mid-Brunhes event might have been a reaction to changes in the global carbon cycle in relation to low-latitude upper ocean changes [15].
Hence, alternative hypotheses in relation to the global carbon cycle need to be considered for the observed Pleistocene δ13C shifts.
Section snippets
Stacked benthic foraminiferal carbon isotope record
In order to assess the Pleistocene benthic foraminiferal δ13C records, we compare several benthic foraminiferal δ13C records (ODP Sites 502, 659, 758, 849, and 1143 [6], [7], [8], [9], [10]) that are all based on the same epibenthic foraminiferal species (Cibicidoides wuellerstorfi) (Fig. 2). Thus possible anomalies associated with infaunal habitats are avoided. The records were selected on the basis of the reliability of the respective age models and continuous coverage (average sample
Results
The changes that are needed in the different parameters in order to establish the observed fluctuations in δ13Cstacked as calculated by the mechanistic box-model after Chuck et al. [23] are summarized in Table 1 for the six scenarios outlined above. Prior to 1.00 Ma, δ13Cstacked variations are presumed to be negligible (Fig. 3), the system is assumed to have been at steady state, and the values used for the various parameters are: δriv = − 3.9‰, δorg = − 21.5‰, Friv = 2.40 × 1014 g C yr− 1, Forg = 4.8 × 1013 g
Which scenario fits best?
In scenario 1, relatively large changes in δriv are needed if they are to drive the δ13Cstacked fluctuations (Table 1). For example, between 0.90 and 0.50 Ma (node 2 and 3), δriv would have to increase from − 4.65 to − 3.60‰. If changes in δriv were the result of changes in the relative weathering rates of inorganic and organic carbon, one would expect this to be reflected in the osmium isotope composition of seawater, as osmium isotopes are sensitive to the ratio of organic carbon weathering to
Discussion
The assessment of the various scenarios outlined above indicates that it is most likely that the observed δ13Cstacked fluctuations were driven by carbon burial changes (scenario 6). Although it cannot be excluded that productivity variations also took place simultaneously, these likely were of relatively minor importance, given that large-scale productivity changes would cause an offset between surface and deep δ13C records, which is not supported by observations (Fig. 6, Supplementary
Summary
Carbon isotope records from both surface- and bottom dwelling foraminifera from all major ocean basins show two long-term δ13C fluctuations between 1.00 and 0.10 Ma. Existing evidence and various hypotheses for these fluctuations are evaluated using the 3-box model of Chuck et al. [23]. Our modeling results suggest that the two long-term δ13C fluctuations most likely resulted from concomitant changes in the burial fluxes of organic and inorganic carbon. In order to avoid unrealistic changes in
Acknowledgements
This study used data provided by the Ocean Drilling Program (ODP). ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc. R. Tiedemann is thanked for making carbon isotope data from Ocean Drilling Program (ODP) Site 659 available. We acknowledge financial support from NERC and the EU program and thank J. Thomson for stimulating discussions. This manuscript was improved through the suggestions of
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2015, Global and Planetary ChangeCitation Excerpt :Middle and dark gray tone sediments could reflect a generalized good bottom and pore-water oxygen conditions and higher abundance in the organic matter content at the surface but also in the first centimeters of the sediment, allowing bioturbation by shallowest, shallow and middle tiers tracemakers. When input of organic matter content (as indicated by Pexp) is maintained during a comparatively long time (Intervals F, or H to K), deep tier traces, i.e., Zoophycos, can be developed, probably reflecting a comparatively higher organic matter content and a slight decrease in oxygenation probably related to the presence of the poorly ventilated and benthic δ13C-deplected Antarctic bottom water AABW (Adkins et al., 2005; Hoogakker et al., 2006) (Fig. 6b). The benthic foraminifer concentration in the sediments and variations of the planktonic foraminifer assemblages suggest significant changes in surface water productivity and food supply to the sea floor occurring in the Portuguese margin during MIS 12 and MIS 11 that could be correlated with the registered changes in facies and trace fossil assemblages (Fig. 6).