Archive for forams

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)

Deep ocean carbon reconstruction helps decipher a million-year-old climate mystery

Posted by mmaheigan 
· Tuesday, July 23rd, 2019 

Approximately one million years ago, Earth’s periodic ice ages increased in strength and duration, shifting from a 41,000-year pacing to a 100,000-year pacing, both linked to Earth’s orbital variations. The causes of this climate shift known as the mid-Pleistocene transition (MPT) have been debated for decades.

A recent study in Nature Geoscience addresses how the ocean carbon cycle contributed to the MPT by quantifying the carbon inventory of the deep Atlantic Ocean during this time. Using trace element and isotope ratios of fossil marine foraminifera, the authors demonstrate that an abrupt weakening of deep ocean overturning circulation between 950,000 and 900,000 years ago occurred alongside a pronounced increase in carbon content of the deep Atlantic Ocean. This study revealed significantly higher carbon concentrations in the deep North and South Atlantic basins during the post-MPT 100,000-year ice ages relative to the 41,000-year ice ages prior to the MPT (Figure 1).

Figure 1 caption: The last two million years of glacial cycles, with present day on left and age increasing from left to right. Orange data are from 41,000-year ice ages; blue data are from,100,000-year ice ages. (A) Glacial-interglacial cycles demonstrated in benthic oxygen isotopes (green), with warmer interglacials up and peak ice ages downward. (B) Atmospheric CO2 from ice core measurements (gray lines) and reconstructed from boron isotopes (circles) (C), Peak ice age neodymium isotope ratios indicating strength of density-driven deep ocean circulation (squares and triangles indicate two different sediment cores). (D) Peak ice age deep ocean carbon content (squares and diamonds indicate two independent reconstructions from the same South Atlantic sediment core).

These data indicate that since 950,000 years ago, the deep Atlantic Ocean has stored an extra 50 billion tons of carbon during peak ice ages. This study hypothesizes that this extra carbon was sequestered from the atmosphere via a feedback between Antarctic ice sheet extent and the efficiency of air-sea carbon exchange in the Southern Ocean. The authors propose that intensification of ice ages one million years ago was closely linked to enhanced ocean carbon storage and resultant lowering of atmospheric CO2 levels.

While paleoclimatologists consider the MPT to be the most recent major climate transition, the magnitude of carbon perturbation at the MPT pales in comparison to today’s human emissions. Today, humans produce 50 billion tons of carbon in only five years. Studies of the carbon cycle across past climate transitions like the MPT provide key insights on how future climate may respond to today’s carbon cycle disruption.

 

Authors
Jesse Farmer (LDEO Columbia University; now at Princeton University and Max Planck Institute for Chemistry)
Bärbel Hönisch, Laura Haynes, Maureen Raymo, Steven Goldstein, Maayan Yehudai, Joohee Kim (LDEO Columbia University)
Heather Ford (Queen Mary University of London)
Dick Kroon, Simon Jung, Dave Bell (University of Edinburgh)
Maria Jaume-Seguí, Leopoldo Pena (University of Barcelona)

 

See this related popular article and video in the Washington Post.

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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