During the last glacial termination the rise in atmospheric pCO₂ was accompanied by a precipitous drop in surface ocean ∆¹⁴C (radiocarbon activity) (Figure below) that cannot be explained by changes in ¹⁴C production alone and therefore requires there to have been a flux of ¹⁴C-depleted carbon into the surface ocean and atmosphere. The long-standing paradigm in Paleoceangraphy is that both the rise in atmospheric pCO₂ and the drop in ∆¹⁴C during the deglaciation was caused by a release of 'old' respired metabolic carbon that had accumulated in the deep sea during the long glaciation. However, over the past decade a number of important new discoveries have been made that are shedding new light on the processes responsible for the drop in ∆¹⁴C and the rise in atmospheric pCO₂. Paleoceanographers have documented large ∆¹⁴C excursions in marine sediments that were deposited in the ocean durign the last glacial termination. These ∆¹⁴C excurions coincided with the rapid drop in atmospheric ∆¹⁴C (Figure below). Thus far these marine ∆¹⁴C excursions have been documented at sites in the eastern equatorial Pacific (EEP), the Arabian Sea, the Baja margin and in the South Pacific, including sites on the Chatham Rise and Bounty Trough near New Zealand. The magnitude of the ∆¹⁴C excursion in places such as the eastern equatorial Pacific (Stott et al, 2009; Stott and Timmermann, 2011; Stott et al., 2019) cannot be reconciled with a release of metabolic carbon. This is because if the radiocarbon ages in the EEP cores reflected 'true' ventilation age of Pacific Intermediate waters, those waters would have been more than 6000 years old during the last deglaciation. And if that had been true, those waters would have been anoxic and there is no evidence of anoxia in the EEP during the last deglaciation.

Figure above is taken from Broecker and Barker (2007) showing the large -190‰ drop in Δ¹⁴C during the last deglaciation.

 

      Recent discoveries have also revealed large accumulations of liquid CO₂ in the sediments blanketing the margins of active submarine volcanic centers in the Pacific Ocean. This is old (¹⁴C-dead) carbon that has risen to the seafloor from deep geologic reservoirs. The liquid CO₂ is held within the surface sediments by a cap of CO₂ hydrate that forms at the sediment/water interface (see Figure below from Lupton et al., 2008). One spectacular example of these accumulations was caught on film by a crew investigating an active site near the Okinawa Trough (see video below). Note the bubbles streaming from the seafloor. These are bubbles of liquid CO₂

Figure from Lupton et al 2008

Video of a lake of liquid CO₂ in the western Pacific (video from Inagaki et al., 2006)

 

The liquid CO₂ and CO₂-rich fluids are produced at hydrothermal centers where CO₂ released at depth migrates upwards through the structural conduits where it reacts with seawater.

Click on the video image from Inagaki et al., 2006 (above) to see a how a reservoir of liquid CO₂ is trapped within sediments beneath a cap of hydrate-CO₂ located in the Back Arc Basin of the Okinawa Trough. When the hydrate cap is punctured the bouyant liquid CO₂ is released to the overlying water column.

Hydrothermal Carbon Release in the Equatorial Pacific During the Deglaciation

We are exploring the possibility that hydrothermal sources of CO₂ contributed to glacial/interglacial CO₂ variability and to the Δ¹⁴C variations during the last deglaciation.

 

The Stott and Timmermann (2011) hypothesis was motivated by the discovery of liquid CO₂ reservoirs such as the ones illustrated above and also by large radiocarbon (¹⁴C) age anomalies in the EEP (Figure below). In the EEP the intermediate depth waters became anomalously old (increased Benthic-Planktic ¹⁴C age difference) during the late glacial and deglaciation.

Figure above shows the ¹⁴C age differene between surface dwelling foraminifers and bottom dwelling foraminifers from three cores collected at intermediate water depths in the EEP. The B-P ¹⁴C age difference would normally be a measure of the age of intermediate waters flowing through the EEP. In the modern ocean the B-P ¹⁴C age difference is about 600 years. However, in these cores the B-P ¹⁴C age increased to more than 6000 years during the deglacial section. If these ¹⁴C ages were a true measure of the age of intermediate waters (ventilation age), those waters would have been anoxic. There is no evidence that the waters were anoxic. Consequently, the anomalously old ages imply there was an input of very 'old' carbon to the EEP during the deglaciation.

We also find an increased accumulation of hydrothermal metals during the late glacial and deglacial section of the cores. There is also geochemical and physical evidence of a drop in carbonate ion concentrations within the water column. And there was variable dissolution of the carbonate during the deglaciation. All of the lines of evidence point to a release of hydrothermal carbon into the water column of the EEP during the last glacial termination that would have lowered the saturation state of carbonate and caused the large excursion in Δ¹⁴C. Importantly, this carbon would have also ventilated to the atmosphere because the EEP is one of the primary conduits for CO₂ exchange between the ocean and atmosphere..

We are investigating the hypothesis put forth by Stott and Timmermann (2011) that as the oceans warmed during deglaciation, the CO₂-hydrate stability horizon would have deepened causing a transient release of ¹⁴C-depleted CO₂ from sediment reservoirs.

 

Using a transient glacial-interglacial simulation conducted with the earth system model of intermediate complexity LOVECLIM we estimate ~3^oC temperature increase at intermediate water depths in the Pacific during the last deglaciation would have been large enough to lower the hydrate stability horizon by several hundred meters and significantly reduce the areal extent of sea floor where hydrate was stable.  This hypothesis, while provocative, would explain why ¹⁴C ages of abyssal water masses were not anomalously old during the last glacial and why there was a large radiocarbon activity (Δ¹⁴C) anomaly during the last deglaciation at intermediate (500-700m) water depths. If correct, this hypothesis implies a total carbon flux from hydrothermal systems significantly larger than is currently estimated.

Figure above shows the 3-phase stability fields for CO₂ in the ocean. Today hydrate CO₂ is stable at water depths below about ~700m and at temperatures below 9^oC (red line). During the last glacial maximum the stabiliy of hydrate would have been as much as 300m shallower (blue line) and thereby greatly expanded the area where hydrate CO₂ would have been stable. The increased area of the ocean seafloor over which hydrate was stable would have provided an expanded resevoir for the accumulation and storage of carbon. Upon deglacial warming, the hydrate stability horizon would have deepened and thereby released the more bouyant liquid CO₂ from sediments to the overlying water column.

Our most recent paper summarizes our findings to date (Stott et al., 2019)

 

Geologic Carbon Release from Pockmarks on Chatham Rise During the Deglaciation

We are also investigating evidence for release of geologic carbon from the pockmarks that occur across a vast region of the Chatham Rise and Bounty Trough in the southwestern Pacific (Figure below). The pockmarks are depression in the surface where sediment has been removed. Pockmarks of this kind are seen in other parts of the ocean and have typically been found to occur where methane clathrate exists within the sediment.

The figure above shows the surface topography of the southwestern margin of the Chatham Rise that is covered with pockmarks.

In 2013 a research cruise to Chatham Rise collected a suite of cores from within and next to pockmarks to test the hypothesis that the pockmarks formed with methane clathrate destabized and vented to the ocean, causing the pockmarks. However, the pore waters that were extracted from cores collected acros s the Chatham Rise contained no evidence of methane. We therefore have begun to test the hypothesis that CO₂ release caused the pockmarks rather than methane. Our initial findings indicate there were large Δ¹⁴C anomalies during the deglaciation that appear to coincide with the timing of pockmark formation (Figure below). Evidence of CO₂ release comes from measuring the ¹⁴C age of benthic (bottom-dwelling) foraminifers extracted from sediment cores collected next to the pockmarks. The large excursions found in these cores (Figure below) point to a release of old ¹⁴C-dead carbon. The most likely source of this carbon would be dissociated limestone that was subducted beneath the Chatham Rise during the Late Cretaceous.

We have just published a paper on our findings Stott et al (2019).

 

citations:

Broecker, W., and Barker, S., 2007, A 190‰ drop in atmosphere's Δ¹⁴C during the "Mystery Interval" (17.5 to 14.5 kyr): Earth and Planetary Science Letters, v. 256, no. 1-2, p. 90-99.

Inagaki, F., Kuypers, M. M. M., Tsunogai, U., Ishibashi, J.-i., Nakamura, K.-i., Treude, T., Ohkubo, S., Nakaseama, M., Gena, K., Chiba, H., Hirayama, H., Nunoura, T., Takai, K., Jørgensen, B. B., Horikoshi, K., and Boetius, A., 2006, Microbial community in a sediment-hosted CO₂ lake of the southern Okinawa Trough hydrothermal system: Proceedings of the National Academy of Sciences, v. 103, no. 38, p. 14164-14169.

Lupton, J., Butterfield, D., Lilley, M., Evans, L., Nakamura, K.-i., Chadwick, W., Jr., Resing, J., Embley, R., Olson, E., Proskurowski, G., Baker, E., de Ronde, C., Roe, K., Greene, R., Lebon, G., and Young, C., 2006, Submarine venting of liquid carbon dioxide on a Mariana Arc volcano: Geochem. Geophys. Geosyst., v. 7.

Stott, L., and Timmermann, A. Hypothesized Link Between Glacial/Interglacial Atmospheric CO₂ Cycles and Storage/Release of CO₂-Rich Fluids From Deep-Sea Sediments. In: Abrupt Climate Change: Mechanisms, Patterns, and Impacts Geophysical Monograph Series 193
Copyright 2011 by the American Geophysical Union. 10.1029/2010GM001052

2019 Shao, J., Stott, L. D., Gray, W. R., Greenop, R., Pecher, I., Neil, H. L., Coffin, R., Davy, B., Rae, W. B Atmosphere-Ocean CO2 Exchange Across the Last Deglaciation from the Boron Isotope Proxy. Paleoceanography/Paleoclimatology. doi: org.libproxy2.usc.edu/ 10.1029/2018PA003498

2019 Stott, L. D., Harazin, K. M., and Quintana Krupinski, N. B. Hydrothermal carbon release to the ocean and atmosphere from the eastern equatorial Pacific during the last glacial termination, Environmental Research Letters, 14(2), 025007, doi: 10.1088/1748-9326/aafe28.

2019 Stott, L. D., Davy, B., Shao, J., Coffin, R., Pecher, I., Neil, H., Rose, P., and Bialas, J., CO₂ Release from Pockmarks on the Chatham Rise-Bounty Trough at the Glacial Termination. Paleoceanography/Paleoclimatology, doi: 10.1029/2019PA003674

2019 Stott, L. D., Deep sea carbon reservoirs once superheated the Earth-could it happen again? The Conversation. https://theconversation.com/deep-sea-carbon-reservoirs-once-superheated-the-earth-could-it-happen-again-113518.