The University of Texas Arlington

blue earth

Department of Earth and Environmental Sciences

  
 
 
 
 


Background

Objectives

Permian Ocean Circulation

Anoxia in the Permian Deep Sea

Reorganization of the Carbon Cycle

Papers Published

References

 

 

 

Background

The last great transition of the Earth from a cold to a warm climate began around 280 million years ago (Ma), with the final melting of the major southern hemisphere ice sheets of the Late Paleozoic. This marked the onset of warm-climate conditions that lasted ~ 250 million years until the beginning of a cooling trend that culminated in the current Late Cenozoic glacial regime. The circumstances causing the onset of the warm period, the changes within the warm period, and the beginning of the cooling, are the subject of intense current research. Factors being considered include: changes in land/ocean distribution, height and location of mountains, ocean seaways, atmospheric greenhouse gas concentration, terrestrial vegetation, terrestrial/marine biogeochemistry, and the intensity of solar radiation (see Barron and Fawcett, 1995; Crowley, 2000; Crowley and Berner, 2001, for an overview of the role of many of these factors in climate change). The investigation of this long period of geologic time is of great interest to studies of earth system dynamics. The mass extinction that occurred at the Permian– Triassic (P–Tr) boundary (251 Ma) represented the most extensive species loss of the last 550 million years (Erwin, 1993). Estimates of extinction are about 90% for marine and 70% for terrestrial organisms (Raup and Sepkoski, 1982; Erwin, 1994; Sepkoski, 1995). Several hypotheses have been discussed to explain the complexity of the P–Tr mass extinction. Most hypotheses associate the mass extinction with (1) the long-term reorganization of the carbon cycle, (2) the release of methane stored in methane hydrates, (3) loading of the ocean with toxic gases, (4) large-scale volcanism, or (5) extraterrestrial impacts by asteroids or comets. Top

Objectives

The reorganization of the carbon cycle during the Permian-Triassic boundary has been inferred from significant excursions in carbon isotope measurements of marine sediments (Erwin et al., 2002; Payne et al., 2004). The mechanisms that caused these isotopic shifts are still controversial. The extent of anoxia in bottom waters of the Permian Panthalassic Ocean, and its role in carbon-cycle anomalies, are not well understood. Also uncertain is the character of the Permian ocean circulation, which has been interpreted in various studies as ranging from sluggish to vigorous. The long-term goal of this proposal is to study aspects of changes in the accumulation of carbon in marine sediments as a consequence of changes in the thermohaline circulation and vertical particle flux during the Permian- Triassic boundary interval. We are planning to couple a marine sediment diagenesis model to the NCAR climate system model (Kiehl and Shield, 2005). We will apply this model to address the following main
questions:

1) How may changes in oceanic circulation, atmospheric composition, carbon inventory, and marine productivity have affected the accumulation of organic and inorganic
carbon in sediments at the Permian-Triassic boundary?

2) How may changes in depositional processes (e.g., particle aggregation and composition) have influenced the geochemical composition of marine sediments?

Permian Circ

Figure 1, Simulated ocean circulation during the Late Permian (see Winguth et al). Click here for comparison of Permian ocean simulations obtained from other three-dimensional general circulation models., 2002, for details) Top

Permian Ocean Circulation

During the Kazanian (Middel Permian, 267 million years ago) the Pangaean superocean was surrounded by a superocean -"Panthalassa". An ocean general circulation model is coupled to an atmospheric energy balance model to simulate the Kazanian climate (Figure 1). Wind fields and freshwater fluxes are taken from an output of an atmospheric general circulation model are taken as initial and boundary conditions. The ocean circulation is dominated by a strong circulation in the Southern hemisphere to balance the stronger air temperature gradients in this hemisphere (Figure 1). The model simulates strong subtropical gyres, which are similar to the modern South Pacific circulation, and consistent to geological evidence from climate-biomes reconstructions. Also, high latitude simulated sea surface temperatures are generally in good agreement with the geological evidence (Figure 2).

Permian PPr Extract figa

Figure 2. (a) Changhsingian (Late Tatarian, Late Ochoan) climate-sensitive sediments and reconstructed water masses from Ziegler et al. (1998).Water mass classification derived from
the climate simulation with 4xCO2.

Figure 2. (b) 8xCO2 with freshwater input in the southern hemisphere (c), and methane release experiment equivalent to 18CO2 concentration (d) using
the classification scheme of Table 2 in Winguth et al. (2002). Top

Anoxia in the Permian Deep Sea

In addition, we study aspects of changes in the marine carbon cycle by changes in thermohaline circulation during one of the most severe mass extinctions in marine and terrestrial biota that occurred at the Permian-Triassic boundary (~251 million years ago). How the mass extinction affected global productivity is still controversial. A reorganization of the carbon cycle associated with a decrease in the marine productivity and an anoxic deep Permian ocean (Panthalassa) has been invoked as one of the possible key mechanisms to explain the widespread occurrence of organic-rich layers. Another popular theory is associated with a massive release of methane from marine sediments. In our study, an atmosphere-ocean circulation model of intermediate complexity coupled to a marine carbon cycle model that includes sediment geochemistry will be used to explore how these changes affect feedback processes between the thermohaline circulation and the carbon cycle.

Permian Image 2

We expect that the comparison of our sensitivity simulations with recent stratigraphic findings (Figure 2 and Figure 3) and data from carbon and sulfur isotopes will lead to an improved understanding of the causes of burial of carbon and an oxygen-depleted environment in the deep Panthalassa. In addition, the analyses of the data and simulations will be beneficial to an improved understanding of the causes of warm climates in earth history, including how earth system processes operate in a warmer world. These results should provide a useful context for understandingpast changes in biodiversity, as well as possible future changes in the earth system, and will lead to a better understanding of the discrepancies between data and climate simulations.

Reorganization of Carbon Cycle Fig 1

Figure 4. Composite chart of the Permian–Triassic interval (269.5–241.0 Ma) from Weidlich et al. (2003) including modeled atmospheric oxygen level, second-order sea level reconstruction, and ecosystem collapse and recovery patterns on land and in sea. Darker shading denotes greater intensity of ocean anoxia. PAL is present atmospheric oxygen level (20.9%). Top

Reorganization of the carbon cycle

Broecker and Peacock (1999) suggested that the negative excursion of 2–4x in the carbon isotopic composition (d13C) of marine carbonates and of organic matter at the P–Tr boundary (Baud et al., 1989; Veizer et al., 1999; Sephton et al., 2002; Payne et al., 2004) may be explained by changes in the global ecosystem induced by the massive mass extinctions. Even higher positive and negative excursions in d13C of up to 8x have been reported for the Early Triassic over time scales of tens to thousands of years (Payne et al., 2004). The continental ecosystem responsible for the burial of organic matter was disrupted and the relatively efficient marine food web of the Permian changed into a less effective Triassic marine food web. Thus, main burial of organic matter probably moved to the marine environment. The rapid change in ocean chemistry is also evident from a dramatic change in sulfur isotopes (e.g. Holser, 1977; Strauss, 1997, 1999). The secular changes could be related to the redox balance in the sulfur cycle. The observed shift of about 5x in the sulfur isotopes (d34S) from sulfates in evaporites may be explained by a change in the mass of net pyrite formation through bacterial sulfate reduction and burial in the sedimentary reservoir (see Strauss,1997, for a review). The greater amount of marine organic matter deposited at the sea floor at the P–Tr boundary is associated with an increase of anoxic bottom waters identified by black, carbon- and sulfide-rich sediments (Isozaki, 1997; Wignall and Twitchett, 2002). It has been speculated that the anoxia in the Panthalassa oceans were associated with a stagnant or sluggish ocean circulation (Knoll et al., 1996; Ryskin, 2003; Beauchamp and Baud, 2002). Stratified deep-sea water masses, like in the modern Black Sea, are favorable for anoxic conditions. However, the Neothethyan ocean along the east coast of southern Pangea may have been welloxygenated by a warm saline deep water formation similar to the Mediterranean Sea (Wignall and Newton, 2003). But saline and warm water would also reduce the solubility of oxygen and would lead to a reduced oxygen concentration in the deep water (solubility ratio between a 0 C and 24 C water mass is ~ 1.6). In this project, we are planning to reevaluate the changes in the deep-sea oxygen concentration with several numerical sensitivity experiments. Top

Papers published

A. M. E. Winguth, E. Maier Remier 2005
Causes of the marine productivity and oxygen changes associated
with the Permian–Triassic boundary: A reevaluation with ocean
general circulation models

A. M. E. Winguth, C. Heinze, J. E. Kutzbach, E. Maier-Reimer, U. Mikolajewicz, D. Rowley, A. Rees, and A. M. Ziegler, 2002
Simulated warm polar currents during the middle Permian

Other References:

Payne, J.L., D.J. Lehrmann, J. Wei, M.J. Orchard, D.P. Schrag, and A.H.
Knoll, 2004. Large perturbations of the
carbon cycle during recovery from the end-Permian extinction. Science, 305,
506-509.

Isozaki, Y., 1997. Permo-Triassic boundary superanoxia and stratified
superocean: Records from lost deep sea.
Science, 276, 235-238.

Erwin, D.H., S.A. Bowring, and J. Yugan, 2002. End-Permian mass extinctions:
A review, In "Catastrophic events
and mass-extinctions: Impacts and beyond", Koerbl, C., and MacLeod, K.G.
(eds.), Geological Society of
America Special Paper, 356, 363-383. Top

 

 
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