Archive for ice age

Air-sea gas disequilibrium drove deoxygenation of the deep ice-age ocean

Posted by mmaheigan 
· Thursday, March 18th, 2021 

During the Last Glacial Maximum (~20,000 years ago, LGM) sediment data show that the deep ocean had lower dissolved oxygen (O2) concentrations than the preindustrial ocean, despite cooler temperatures of this period increasing O2 solubility in sea water.

Figure 1. a) Whole ocean inventory of the O2 components in the preindustrial control (PIC): total O2 (O2); the preformed components equilibrium O2 (O2 equilibrium), physical disequilibrium O2 (O2 diseq phys) and biologically-mediated disequilibrium (O2 diseq bio); and O2 respired from soft-tissue (O2 soft). b) The difference in whole ocean inventory of O2 components between the LGM and PIC simulations.

In a study published in Nature Geoscience, the authors provide one of the first explanations for glacial deoxygenation. The authors combined a data-constrained model of the preindustrial (PIC) and LGM ocean with a novel decomposition of O2 to assess the processes affecting the oceanic distribution of oxygen. The decomposition allowed for the preformed disequilibrium O2—the amount of oxygen that deviates from its solubility equilibrium value when at the surface—to be tracked, along with other contributions such as the O2 consumed by bacterial respiration of organic matter. In the preindustrial ocean, a third of the subsurface oxygen deficit was a result of disequilibrium rather than oxygen consumed by bacteria. This contradicts previous assumptions (Figure 1a). Nearly 80% of the disequilibrium resulted from upwelling waters, depleted in O2 due to respiration, not fully equilibrating before re-subduction into the ocean interior. This effect was even greater during the LGM (Figure 1b). The authors attributed this largely to the widespread presence of sea ice—which acts as a cap on the surface preventing the water from gaining oxygen from the atmosphere—in the ocean around Antarctica, with a smaller contribution from iron fertilization.

This study provides one of the first mechanistic explanations for LGM deep ocean deoxygenation. As the ocean is currently losing oxygen due to warming, the effect of other processes, including sea ice changes, could prove important for understanding long-term ocean oxygenation changes.

Authors
Ellen Cliff (University of Oxford)
Samar Khatiwala (University of Oxford)
Andreas Schmittner (Oregon State University)

Joint highlight with GEOTRACES International Project Office

A role for tropical nitrogen fixers in glacial CO2 drawdown

Posted by mmaheigan 
· Wednesday, December 4th, 2019 

Iron fertilization of marine phytoplankton by Aeolian dust is a well-established mechanism for atmospheric carbon dioxide (CO2) drawdown by the ocean. When atmospheric CO2 decreased by 90-100 ppm during previous ice ages, fertilization of iron-limited phytoplankton in the high latitudes was thought to have contributed up to 1/3 (30 ppm) of the total CO2 drawdown. Unfortunately, recent modeling studies suggest that substantially less CO2 (only 2-10 ppm) is sequestered by the ocean in response to high latitude fertilization.

The limited capacity for high latitude CO­2 sequestration in response to iron enrichment motivated the authors of a new study published in Nature Communications to address how lower latitude phytoplankton could contribute to CO2 drawdown. The authors used an ocean model to show that in response to Aeolian iron fertilization, dinitrogen (N2) fixers, specialized phytoplankton that introduce bioavailable nitrogen to tropical surface waters, drive the sequestration of an additional 7-16 ppm of CO2 by the ocean.

Figure 1: Scenarios of Fe supply to the tropical Pacific. In the low iron scenario, analogous to the modern climate, N2 fixation (yellow zone and dots) is concentrated in the Northwest and Southwest subtropical Pacific where aeolian dust deposition is greatest. Non-limiting PO4 concentrations (green zone and dots) exist within the tropics and spread laterally from the area of upwelling near the Americas and at the equator (blue zone). In the high Fe scenario, analogous to the glacial climate, N2 fixation couples to the upwelling zones in the east Pacific, enabling strong utilisation of PO4, the vertical expansion of suboxic zones (grey bubbles) and a deeper injection of carbon-enriched organic matter (downward squiggly arrows).

These results provide evidence of a tropical ocean CO2 sequestration pathway, the mere existence of which is hotly debated. Importantly, the study describes an additional mechanism of CO2 drawdown that is complementary to the high latitude mechanism. When combined, their contributions elevate iron-driven CO2 drawdown towards the expected 30 ppm, making iron fertilization a driver of a stronger biological pump on a global scale.

 

Authors:
Pearse Buchanan (University of Liverpool, University of Tasmania, CSIRO Oceans and Atmosphere, ARC Centre of Excellence in Climate System Science)
Zanna Chase (University of Tasmania)
Richard Matear (CSIRO Oceans and Atmosphere, ARC Centre of Excellence in Climate Extremes)
Steven Phipps (University of Tasmania)
Nathaniel Bindoff (University of Tasmania, CSIRO Oceans and Atmosphere, ARC Centre of Excellence in Climate Extremes, Antarctic Climate and Ecosystems Cooperative Research Centre)

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.

Antarctic Ocean CO2 helped end the ice age

Posted by mmaheigan 
· Tuesday, April 2nd, 2019 

Many scientists have long hypothesized that the ocean around Antarctica was responsible for changing CO2 levels during ice ages, but lacked definitive evidence. A new study in Nature provides the most direct evidence of this process to date and provides crucial evidence of the mechanisms—including changing sea ice cover and bipolar seesaw (warming in the Southern Hemisphere during cooling in the Northern Hemisphere) events—that controlled CO2 and climate during the ice ages.

Using samples of fossil deep-sea corals collected from 1000 m in the Drake Passage (Figure 1a), the authors were able to reconstruct the CO2 content of the deep ocean. They found that the deep ocean CO2 record was the “mirror image” of CO2 in the atmosphere (Figure 1b), with the ocean storing CO2 during an ice age and releasing it back to the atmosphere during deglaciation. CO2 rise during the last ice age occurred in a series of steps and jumps associated with intervals of rapid climate change.

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As well as helping scientists better understand the ice ages, the new findings also provide context to current CO2 rise and climate change. Although the CO2 rise that helped end the last ice age was dramatic in geological terms, CO2 rise due to human activity over the last 100 years is even larger and about 100 times faster. CO2 rise at the end of the ice age helped drive major melting of ice sheets resulting in sea level rise of >100 meters. These results bolster the idea that if we want to prevent dangerous levels of global warming and sea level rise in the future, we need to reduce CO2 emissions as quickly as possible

Authors:
J. W. B. Rae (University of St Andrews, UK)
A. Burke (University of St Andrews, UK)
L. F. Robinson (University of Bristol, UK)
J. F. Adkins (California Institute of Technology)
T. Chen (University of Bristol, UK, Nanjing University, China)
C. Cole (University of St Andrews, UK)
R. Greenop (University of St Andrews, UK)
T. Li (University of Bristol, UK, Nanjing University, China)
E. F. M. Littley (University of St Andrews, UK)
D. C. Nita (University of St Andrews, UK, Babes-Bolyai University, Romania)
J. A. Stewart (University of St Andrews, UK, University of Bristol, UK)
B. J. Taylor (University of St Andrews, UK)

Constraints on glacial overturning circulation and export production lead to answers about the carbon cycle

Posted by mmaheigan 
· Friday, January 4th, 2019 

One of the biggest unsolved mysteries in climate science concerns the dynamics and feedbacks of the ice age carbon dioxide (CO2) cycle.

At the height of the Pleistocene ice ages, the atmospheric CO2 concentration was about 1/3 lower than during the warm interglacial periods. Most scientists think that the CO2 that was missing from the atmosphere was in the deep ocean, but how and why remains unclear. In a study published in Earth and Planetary Science Letters, we compared different computer simulations of the ice age ocean with δ13C, radiocarbon (14C), and δ15N data from sea floor sediments.

We find that a weak and shallow Atlantic Meridional Overturning Circulation (6-9 Sv, or approximately half of today’s overturning rate) best reproduces the glacial sediment isotope data. Increasing the atmospheric soluble iron flux in the model’s Southern Ocean intensifies export production, carbon storage, and further improves agreement with glacial δ13C and δ15N reconstructions.

Figure Caption: Depth profiles of global mean δ13C, calculated using only grid boxes for which there exists Last Glacial Maximum data. Blue: Weak Atlantic circulation; Red: Strong Atlantic circulation; Green: Collapsed Atlantic circulation; Dashed: Extra iron in the Southern Ocean; Orange: Last Glacial Maximum Data.

Our best-fitting simulation (blue, dashed line in the figure) is a significant improvement over previous studies and suggests that both circulation and export production changes were necessary to maximize carbon storage in the glacial ocean. These findings provide an equilibrium glacial state, consistent with a combination of proxies, that can be used as a basis for simulations covering the last deglaciation time period. Understanding the different states that the global climate system can transit, and the characteristics of the transitions, is crucial to project possible outcomes of current climate change processes.

 

Authors:
Juan Muglia (Oregon State University)
Luke C. Skinner (Godwin Laboratory for Palaeoclimate Research, University of Cambridge)
Andreas Schmittner (Oregon State University)

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