CO2 sigma 13 ratios, again…

There’s a lot of folks claiming that the ONLY way we could have C-13 depletion in the atmosphere is from humans burning coal and oil and putting all that C-12 into the air. This, of course, ignores that we don’t really know what the C isotope ratio was in all the coal and oil we have burned. It is variable by source, and is not all the same. I covered that in an earlier posting here:

https://chiefio.wordpress.com/2009/02/25/the-trouble-with-c12-c13-ratios/

and a bit more here:

https://chiefio.wordpress.com/2012/04/29/about-those-co2-isotopes/

As an example of the kind of “must be people what done it” article, there is this one:

http://www.wired.com/2015/04/volcanic-versus-anthropogenic-carbon-dioxide-addendum/

There is really no way around it. Since the dawn of the industrial age, humans have taken carbon locked in organic material and released it into the atmosphere. That burning added huge volumes of carbon dioxide (in 2014, 44 billion tonnes) that all has highly negative carbon isotopic composition. Carbon dioxide goes up, the carbon isotopic composition goes down, all recoded in the ice at the poles.

One Small Problem… Some nice folks looked into past changes with depletion of C-13 during warm times. Seems that nature manages to deplete the C-13 component of the atmosphere and ocean all by itself when things warm up. No humans needed (or even really around then… it was about 50+ million years ago).

http://wesscholar.wesleyan.edu/cgi/viewcontent.cgi?article=1116&context=div3facpubs

Please forgive any artifacts from the cut / paste from PDF. That often has issues with special symbols. I’ve tried to catch most such things, but really you ought to just read the original. This is just the ‘taster’… The original has nice graphs and charts in it too ;-)

High-resolution deep-sea carbon and oxygen isotope records of Eocene Thermal Maximum 2 and H2

Lucy Stap1, Lucas J. Lourens1, Ellen Thomas2,3, Appy Sluijs4, Steven Bohaty5, and James C. Zachos6

1 Faculty of Geosciences, Utrecht University, 3584 CD Utrecht, Netherlands
2 Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, USA
3 Department of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut 06459, USA
4 Biomarine Sciences, Institute of Environmental Biology, Utrecht University, 3584 CD Utrecht, Netherlands
5 School of Ocean and Earth Science, National Oceanography Centre, University Road, Southampton SO17 1BJ, UK
6 Earth and Planetary Sciences, University of California, Santa Cruz, California 95064, USA

ABSTRACT

Eocene Thermal Maximum 2 (ETM2) and H2 were two short-lived global warming events that occurred ~2 m.y. after the Paleocene–Eocene thermal maximum (PETM, ca. 56 Ma). We have generated benthic foraminiferal stable carbon and oxygen isotope records of four sites along a depth transect on Walvis Ridge (~3.5–1.5 km paleodepth, southeast Atlantic Ocean) and one site on Maud Rise (Weddell Sea) to constrain the pattern and magnitude of their carbon isotope excursions (CIEs) and deep-sea warming. At all sites, ETM2 is characterized by ~3 °C warming and a –1.4‰ CIE. The H2 event that occurred ~100 k.y. later is associated with ~2 °C warming and a –0.8‰ CIE. The magnitudes of the δ13C and δ18O excursions of both events are significantly smaller than those during the PETM, but their coherent relation indicates that the δ13C change of the exogenic carbon pool was similarly related to warming during these events, despite the much more gradual and transitioned onset of ETM2 and H2.
[…]

Now we have two different sets of units to compare, but if I’m reading the chart in that first article correctly, it is saying there has been about a 2 ppm change of CO2 isotope ratio to less C-13. The second link / paper says about a 1% reduction. Now I’ve not scoured the paper to figure out “percent of what”, so can’t say if it is percent of total CO2 or percent of C-13 based CO2, or percent of C-13… but at 400 ppm now, that’s about 4 ppm excursion if it is based on all CO2, (or about 2 ppm if CO2 was lower at 200 ppm then). That’s about the same as the present excursion. Hmmmmm….

RESULTS AND DISCUSSION

ETM2 and H2 are characterized by pronounced and concomitant negative δ13C (respectively –1.4‰ and –0.8‰) and δ18O (respectively –0.8‰ and –0.5‰) excursions at intermediate to abyssal water depths (Fig. 1). The δ13C and δ18O excursions during ETM2 are less pronounced at site 1263 than at the other sites. A similar pattern is depicted in the isotope records of the benthic foraminifera Anomalinoides spp. (Lourens et al., 2005) and O. umbonatus (Fig. DR4). Benthic foraminiferal abundances in the Elmo horizon are exceptionally low, possibly due to an expansion of the oxygen minimum zone, which excluded many species of benthic foraminifera and all large specimens from the relatively shallow depths (site 1263) during the nadir of the excursion. This, combined with bioturbation, may have biased the multispecimen record at this site.

In addition, scanning electron micrographs revealed that the wall structure of N. truempyi is moderately altered by recrystallization and/or secondary calcite throughout the studied interval of all sites (Fig. DR2). We therefore suspect that a larger diagenetic overprint of the primary isotopic signal at the shallower Walvis Ridge sites caused the on-average heavier δ18O values, in particular within ETM2 (Fig. 1).

In general, the oxygen (and carbon) isotopic compositions of benthic foraminifera in deep-sea cores are thought to be less altered by early diagenesis than those of planktic foraminifera (Sexton et al., 2006) and therefore can be used to estimate deep-sea temperatures. The mean δ18O values of the studied sites prior to ETM2 are –0.1‰ to –0.3‰ (Fig. 1), indicating a deep-sea temperature of ~12 °C (Bemis et al., 1998), consistent with the global benthic stack for this time interval (Zachos et al., 2001). The ~0.8‰ drop in δ18O values during ETM2 implies that deep-sea temperatures rose by >3 °C to peak temperatures of 15–16 °C. Single-specimen derived temperatures at site 690 are very similar (Fig. 1), indicating interregional consistency and thus a uniform magnitude of deep-sea warming at these locations and depths during ETM2. The δ18O decrease of ~0.5‰ associated with H2 shows that deepwater temperatures increased by as much as ~2 °C (e.g., from ~12 to 14 °C).

The uniformity between the δ13C and δ18O trends and absolute values for ETM2 and H2 among the different Walvis Ridge sites suggests that the deep-sea water column was homogeneous during the Early Eocene. This observation supports previous inferences from data and modeling studies (Emanuel, 2002; Via and Thomas, 2006) that point to a different structure of the oceans and circulation mode during the early Paleogene than today, i.e., with reduced meridional surface temperature and vertical temperature gradients (e.g., Zachos et al., 1992; Bijl et al., 2009). Site 690 in the Southern Ocean reveals essentially the same δ13C and δ18O patterns and values as the Walvis Ridge sites, suggesting that they were all bathed by a water mass of similar temperature and dissolved inorganic carbon (DIC) isotopic content (δ13CDIC), probably a single Southern Ocean intermediate to deep water mass (Via and Thomas, 2006).

The onsets of ETM2 and H2 are characterized by multiple transitions in δ13C and δ18O (Fig. 1), which covary with the precession-paced steps in bulk carbonate isotope records and carbonate dissolution reported in Stap et al. (2009). From the onset of ETM2 at a relative age of 0 k.y., δ13C and δ18O values dropped by respectively ~1.4‰ and ~0.8‰ along two major transitions

(Fig. 1). The largest excursion in the benthic δ18O and δ13C records occurred during transition 2 of ETM2 (i.e., between 21 and 40 k.y. after the onset).
Transition 3, which coincides with the base of the Elmo horizon (i.e., interval of maximum carbonate dissolution) (Stap et al., 2009), is not marked by a signifi cant shift in benthic δ18O and δ13C values. Remarkably, in contrast to carbonate dissolution patterns (Stapet al., 2009), the recovery phases of the isotope records lack the precession-paced shifts of Stap et al. (2009), suggesting a partial decoupling between changes in deep-sea δ13CDIC and carbonate saturation state during this period.

To assess whether ETM2 shares characteristics with the PETM in terms of carbon cycle and climate, we compared the covariance of benthic foraminiferal δ13C and δ18O values. For ETM2 (including transitions 1 and 2) and H2, Δδ18O/ Δδ13C is roughly 0.8/1.4 and 0.5/0.8, respectively (Figs. 1 and 2). Remarkably, the PETM δ13C and δ18O excursions in N. truempyi and O. umbonatus of –3.5‰ and –2.4‰ at site 1263 (McCarren et al., 2008) reveal an almost identical relation with a slope of 0.69, regardless of their much larger magnitudes (Fig. 2). The coherent relationship in benthic δ13C and δ18O indicates that the δ13C change of the exogenic carbon pool was similarly related to warming during the PETM, ETM2, and H2. Assuming that climate sensitivity did not change significantly during the Early Eocene, this suggests that the isotopic composition of the source(s) of carbon was similar for these events, as suggested previously (Lourens et al., 2005; Nicolo et al., 2007). This can be tested by using the carbonate dissolution horizons to constrain the total mass of carbon released during ETM2 as has been done for the PETM (Ridgwell, 2007; Zeebe et al., 2009). One complicating factor in comparing the δ13C, temperature, and deep-sea carbonate dissolution records for the PETM, ETM2, and H2 is that the latter two events apparently had a much more gradual onset. In particular, several single specimens from the onset of ETM2 at site 690 show δ13C and δ18O values that seem intermediate between the preETM2 and minimum ETM2 data populations (Figs. 1 and DR3). Such intermediate values have not been observed in any PETM section (Thomas et al., 2002; Zachos et al., 2007), even at locations with much higher sedimentation rates (John et al., 2008), though with less robust age control. All available information, hence, suggests that a signifi cant portion of the carbon injection during the PETM occurred in less than 10 k.y., while the entire carbon injection during ETM2 took ~20 k.y. with the current age model at Walvis Ridge (from transition 2 onwards in Fig. 1). Regardless of the apparently different rate of carbon injection during the events, the data imply that the change in global exogenic δ13C was similarly proportional to the degree of warming during the PETM, ETM2, and H2.

So if I’m reading this right, perfectly normal hot excursions happened in the past, and they had CO2 isotope ratios depleted in C-13 all on their own. So anyone asserting that the CO2 isotope ratio MUST be due to human CO2 needs to account for the natural process, or else they are just being ignorant of nature. Is there still room for a human contribution to the isotope ratio change? I don’t know. I’ve not “done the math”. But I’d be surprised if there was not some contribution. But until the actual natural processes are accounted for, it is grossly imprudent to leap to any conclusion.

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