Archive for paleoceanography

Unveiling the Hidden Secrets of Ancient Carbon Burial

Posted by mmaheigan 
· Thursday, August 31st, 2023 

How much carbon has been buried in the depths of our ancient oceans, and how did it shape our planet’s climate? Unraveling this enigma has long eluded researchers, but a recent groundbreaking “bottom-up” study unveils the surprising history of organic carbon burial in marine sediments during the Neogene period.

Departing from conventional methods, this study presents an innovative approach to calculating organic carbon burial rates independently. Drawing from data collected from 81 globally distributed sites, the research covers the Neogene era (approximately 23 to 3 million years ago). The results reveal unprecedented spatiotemporal variability in organic carbon burial, challenging previous estimates. Notably, high burial rates were found during the early Miocene and Pliocene, contrasting with a significant decline during the mid-Miocene, marked by the lowest ratio of organic-to-carbonate burial rates. This finding disputes earlier interpretations of enriched carbonate 13C values during the mid-Miocene (so called “Monterey Period” or “Monterey Excursion”) as indicative of massive organic carbon burial.

Figure Caption: Neogene organic carbon (OC) burial in the global ocean. Burial rates calculated using different definitions of provinces, including three approaches: Longhurst (black curve with uncertainty envelope，± 1σ in purple and ± 2σ in pale lilac), Oceans (blue curve), and FAO Fishing (orange curve).

Understanding the complex carbon burial dynamics of ancient oceans holds profound implications for comprehending our planet’s climate evolution. The suppressed organic carbon burial during the warm mid-Miocene, likely driven by temperature-dependent bacterial degradation, suggests the organic carbon cycle acted as a positive feedback mechanism during past global warming events. These findings emphasize the vital role of ocean carbon sequestration, providing stark evidence for policymakers, funding agencies, citizens, and educators to acknowledge its significance in combating modern climate challenges.

Authors
Ziye Li (University of Bremen)
Yi Ge Zhang (Texas A&M University)
Mark Torres (Rice University)
Ben Mills (University of Leeds)

Twitter: @chemclimatology

Backstory
Dr. Zhang, a shipboard organic geochemist during International Ocean Discovery Program Expedition 363, embarked on the legendary drilling ship JOIDES Resolution. While on the journey, Yige spent hours and hours daily crushing samples to measure organic carbon until his palm grew calluses, but the TOC% numbers did not really change. Fueled by sheer determination, Yige’s former student Ziye Li and himself delved into 50 years of IODP data archives to uncover global trends, and with the help of carbon cycle modelers Mark Torres and Ben Mills, leading to the discovery of the history of organic carbon burial.

Unmixing deep sea sedimentary records identifies sensitivity of marine calcifying zooplankton to abrupt warming and ocean acidification in the past

Posted by mmaheigan 
· Tuesday, May 3rd, 2022 

Ocean acidification and rising temperatures have led to concerns about how calcifying organisms foundational to marine ecosystems, will be affected in the near future. We often look to analogous abrupt climate change events in Earth’s geologic past to inform our predictions of these future communities. The Paleocene-Eocene thermal maximum (PETM) is an apt analog for modern climate change. The PETM was a global warming and ocean acidification event driven by geologically abrupt changes to the global carbon cycle approximately 56 million years ago. Much of what we know about the PETM is from the study of deep sea sedimentary records and the microfossils within them. However, these records can experience intense sediment mixing—from bottom water currents and burrowing by organisms living along the seafloor—which can blur or distort the primary climate and ecological signals in these paleorecords.

PETM corrected foram graphic - see caption for detail

Figure 1. A) Frequency distribution of single-shell stable carbon isotope (δ13C) values for planktic foraminiferal shells from a deep sea sedimentary PETM record from the equatorial Pacific (n = 548). Note that 50% of shells measured record distinctly PETM values, while 49.5% record distinctly pre-PETM values. B) Comparison of diversity metric (Shannon-H) between the isotopically filtered (i.e., unmixed) and unfiltered (i.e., mixed) planktic foraminiferal assemblages.

A recent study in the Proceedings of the National Academy of Sciences used geochemical signatures measured from individual microfossil shells of planktic foraminifera (surface-dwelling marine calcareous zooplankton) to deconvolve the effects of sediment mixing on a deep sea PETM record from the equatorial Pacific. Use of this “isotopic filtering” (unmixing) method revealed that nearly 50% of shells in the PETM interval were reworked contaminants that lived before the global warming event (Figure 1A). The identification and removal of these older shells from fossil census counts drastically changed interpretations of how these organisms responded to the PETM. Prior interpretations assumed that planktic foraminiferal communities living near the equator diversified during the PETM. However, by deconvolving the effects of sediment mixing on the same equatorial deep sea record, researchers found that these communities actually suffered an abrupt decrease in diversity at the onset of the PETM (Figure 1B). This decrease is likely due to several taxa migrating towards the poles to escape the extreme heat of the tropics and lower oxygen conditions found at deeper water depths (i.e., thermocline) during the PETM. Additionally, some taxa may have undergone morphological changes, signaling reduced calcification, in response to extreme acidifying conditions. Today, anthropogenic carbon emission rates are ~10 times faster than the carbon cycling perturbation that triggered the PETM. Although planktic foraminifera and other key zooplankton survived the PETM, these communities suffered at the hands of extreme sea surface temperatures and acidifying waters, and may not be able to cope the rate of environmental changes in our ocean over the coming centuries.

 

Authors:
Brittany N. Hupp (University of Wisconsin-Madison)
D. Clay Kelly (University of Wisconsin-Madison)
John W. Williams (University of Wisconsin-Madison)

Volcanic carbon dioxide drove ancient global warming event

Posted by mmaheigan 
· Thursday, March 29th, 2018 

A study recently published in Nature suggests that an extreme global warming event 56 million years ago known as the Palaeocene-Eocene Thermal Maximum (PETM) was driven by massive CO2 emissions from volcanoes during the formation of the North Atlantic Ocean. Using a combination of new geochemical measurements and novel global climate modelling, the study revealed that atmospheric CO2 more than doubled in less than 25,000 years during the PETM.

The PETM lasted ~150,000 years and is the most rapid and extreme natural global warming event of the last 66 million years. During the PETM, global temperatures increased by at least 5°C, comparable to temperatures projected in the next century and beyond. While it has long been suggested that the PETM event was caused by the injection of carbon into the ocean and atmosphere, the source and total amount of carbon, as well as the underlying mechanism have thus far remained elusive. The PETM roughly coincided with the formation of massive flood basalts resulting from of a series of eruptions that occurred as Greenland and North America started separating from Europe, thereby creating the North Atlantic Ocean. What was missing is evidence linking the volcanic activity to the carbon release and warming that marks the PETM.

To identify the source of carbon, the authors measured changes in the balance of isotopes of the element boron in ancient sediment-bound marine fossils called foraminifera to generate a new record of ocean pH throughout the PETM. Ocean pH tells us about the amount of carbon absorbed by ancient seawater, but we can get even more information by also considering changes in the isotopes of carbon, which provide information about the carbon source. When forced with these ocean pH and carbon isotope data, a numerical global climate model implicates large-scale volcanism associated with the opening of the North Atlantic as the primary driver of the PETM.

 

North Atlantic microfossil-derived isotope records from extinct planktonic foraminiferal species M. subbotinae relative to the onset of the PETM carbon isotope excursion (CIE). The negative trend in carbon isotope composition (A) during the carbon emission phase is accompanied by decreasing pH (decreasing δ11B, panel B) and increasing temperature (decreasing δ18O, panel C). Panels D and E zoom in on the PETM CIE, showing microfossil δ13C (D) and δ11B-based pH (E) reconstructions. Also included in E are data from Penman et al. (2014) on their original age model, with recalculated (lab-based) pH values.

 

These new results suggest that the PETM was associated with a total input of >12,000 petagrams of carbon from a predominantly volcanic source. This is a vast amount of carbon—30 times larger than all of the fossil fuels burned to date and equivalent to all current conventional and unconventional fossil fuel reserves. In the following Earth System Model simulations, it resulted in the concentration of atmospheric CO2 increasing from ~850 parts per million to >2000 ppm. The Earth’s mantle contains more than enough carbon to explain this dramatic rise, and it would have been released as magma poured from volcanic rifts at the Earth’s surface.

How the ancient Earth system responded to this carbon injection at the PETM can tell us a great deal about how it might respond in the future to man-made climate change. Earth’s warming at the PETM was about what we would expect given the CO2 emitted and what we know about the sensitivity of the climate system based on Intergovernmental Panel on Climate Change (IPCC) reports. However, the rate of carbon addition during the PETM was about twenty times slower than today’s human-made carbon emissions.

In the model outputs, carbon cycle feedbacks such as methane release from gas hydrates—once the favoured explanation of the PETM—did not play a major role in driving the event. Additionally, one unexpected result was that enhanced organic matter burial was important in ultimately drawing down the released carbon out of the atmosphere and ocean and thereby accelerating the recovery of the Earth system.

 

Authors:
Marcus Gutjahr (National Oceanography Centre Southamption, GEOMAR)
Andy Ridgwell (Bristol University, University of California Riverside)
Philip F. Sexton (The Open University, UK)
Eleni Anagnostou (National Oceanography Centre Southamption)
Paul N. Pearson (Cardiff University)
Heiko Pälike (University of Bremen)
Richard D. Norris (Scripps Institution of Oceanography)
Ellen Thomas (Yale University, Wesleyan University)
Gavin L. Foster (National Oceanography Centre Southamption)

 

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