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A Permafrost Glacial Hypothesis to Explain Atmospheric Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Clim. Past Discuss., 6, 2199–2221, 2010 www.clim-past-discuss.net/6/2199/2010/ Climate of the Past CPD doi:10.5194/cpd-6-2199-2010 Discussions 6, 2199–2221, 2010 © Author(s) 2010. CC Attribution 3.0 License. A permafrost glacial This discussion paper is/has been under review for the journal Climate of the Past (CP). hypothesis to explain Please refer to the corresponding final paper in CP if available. atmospheric CO2 R. Zech et al. A permafrost glacial hypothesis to explain Title Page atmospheric CO2 and the ice ages during the Pleistocene Abstract Introduction Conclusions References R. Zech1,*, Y. Huang1, M. Zech2, R. Tarozo1, and W. Zech3 Tables Figures 1Geological Sciences, Brown University, Providence, USA 2 Institute of Geomorphology and Department of Soil Physics, University of Bayreuth, J I Bayreuth, Germany 3Institute of Soil Science and Soil Geography, University of Bayreuth, Bayreuth, Germany J I *now at: Geological Institute, ETH Zurich, Zurich, Switzerland Back Close Received: 26 September 2010 – Accepted: 8 October 2010 – Published: 15 October 2010 Full Screen / Esc Correspondence to: R. Zech ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. Printer-friendly Version Interactive Discussion 2199 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract CPD Over the past several 100 ka glacial-interglacial cycles, the concentration of atmo- 6, 2199–2221, 2010 spheric CO2 was closely coupled to global temperature, which indicates the impor- tance of CO2 as a greenhouse gas. The reasons for changes in atmospheric CO2 5 have mainly been sought in the ocean, yet proxy evidence does not support the no- A permafrost glacial tion of increased oceanic carbon storage during glacials. Here we present results from hypothesis to explain the first permafrost loess sequence in Siberia spanning two glacial cycles (∼240 ka), atmospheric CO2 which reveal that permafrost soils repeatedly sequestered huge amounts of terrestrial carbon during glacial periods. This can be explained with permafrost favouring more R. Zech et al. 10 intensive waterlogging conditions and better preservation of soil organic matter. Ter- restrial carbon stored in permafrost soils was released upon warming and provided a powerful feedback mechanism for the glacial terminations. We outline a “permafrost Title Page glacial hypothesis” building on integrated annual insolation forcing, which readily ex- Abstract Introduction plains the observed succession of the ice ages during the Pleistocene, including the 15 mid-Pleistocene transition. Conclusions References Tables Figures 1 Introduction J I Antarctic ice cores have revealed that the succession of glacials and interglacials dur- ing the past ∼1 Ma was intimately linked to the global carbon cycle (Petit et al., 1999; J I Shackleton, 2000; Luthi et al., 2008). Cold glacial periods were characterized by low Back Close 20 concentrations of atmospheric CO2 (∼180 to 200 ppm), while interglacials had high concentrations (∼280 ppm, preindustrial value). The large size of the carbon pool in Full Screen / Esc the ocean (∼40 times the atmospheric carbon) led to the prevailing notion that the oceans were the principal driver and acted as a net carbon sink during the glacials Printer-friendly Version (Broecker, 1982; Sigman et al., 2010). Changes in the Southern Ocean in particular Interactive Discussion 25 have been proposed to play an important role in controlling atmospheric CO2, because up-welling and ventilation of the deep ocean, which constantly accumulates respired 2200 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | CO2 from sinking organic particles, occurs mainly around Antarctica (Toggweiler et al., 2006; Fischer et al., 2010; Sigman et al., 2010). However, it has been impossible so far CPD to find the supposedly large pool of “old” radiocarbon trapped in the glacial deep ocean, 6, 2199–2221, 2010 which would be required to corroborate the “ocean hypothesis” (Broecker and Barker, 5 2007; De Pol-Holz et al., 2010). Additionally, carbon models are yet unable to con- vincingly explain the full range of glacial-interglacial changes in atmospheric CO2 with A permafrost glacial physical and biological changes in the ocean (Brovkin et al., 2007; Boer et al., 2010; hypothesis to explain Fischer et al., 2010). In light of these inconsistencies, we should stay open minded to atmospheric CO2 the possibility of changes in the terrestrial carbon cycle as an alternative. R. Zech et al. 10 Terrestrial carbon pools have long been considered to be negligible in comparison to marine reservoirs, and to have acted as sources rather than sinks during glacials. This partly stems from the fact that net production of terrestrial biomass is lower dur- Title Page ing glacials compared to interglacials, because lower temperatures, lower atmospheric CO2, and increased glacial aridity are less favorable conditions for plant growth. Re- Abstract Introduction 15 vised estimates, however, suggest that the soil carbon pools in northern permafrost regions are much larger than previously thought. They could exceed 1670 Pg (Zimov Conclusions References and Schuur, 2006; Tarnocai et al., 2009), which is more than twice the atmospheric Tables Figures pool. Changes in sizes of these carbon pools could thus have major impacts on atmo- spheric CO2 levels. The enormous carbon storage at high-latitudes reflects that apart J I 20 from biomass productivity, the rate of decomposition is crucial for terrestrial carbon storage. Cold permafrost regions provide very suitable environments for preservation J I of soil organic matter and long-term carbon storage. In fact, many studies have fueled Back Close concerns that today, at times of global warming, CO2 and methane emissions from thawing and decomposing permafrost soils result in a large positive feedback and ac- Full Screen / Esc 25 celerate global warming (Zimov and Schuur, 2006; Schuur et al., 2008; Tarnocai et al., 2009). Printer-friendly Version In order to evaluate the role of permafrost soils in the global carbon cycle, informa- tion is needed about their age, formation, and carbon dynamics. So far, the lack of Interactive Discussion long, continuous outcrops spanning several glacial-interglacial cycles has limited such 2201 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | insights. We here present results from a unique loess-paleosol sequence, which doc- uments environmental and climate changes over the last ∼240 ka. CPD 6, 2199–2221, 2010 2 Material and methods A permafrost glacial Our research site is a ∼50 m high permafrost bluff along the Tumara River in the south- hypothesis to explain 5 western foreland of the Verkhoyansk Mountains, Northeast Siberia (Fig. 1). The lower atmospheric CO part of the bluff exposes Tertiary sands and Quaternary gravels. The upper ∼15 m, 2 referred to as “Tumara Paleosol Sequence”, consist of frozen dark grey loess-like sedi- R. Zech et al. ments alternating with bright brown soil horizons (Fig. 2). Previous detailed stratigraphi- cal and geochemical analyses (Zech et al., 2007, 2008) have shown that the dark units 10 (B and D) are dominated by silt size particles and have total organic carbon (TOC) Title Page contents exceeding 1%. The bright brown units (A, C and E) contain more clay and have much lower TOC (< 0.5%). These findings have been interpreted to document Abstract Introduction the succession of glacials and interglacials, with low temperatures favoring intensive Conclusions References permafrost and water logging conditions and good preservation of soil organic matter, 15 while warmer temperatures favor better drainage, mineralization and weathering (Zech Tables Figures et al., 2007, 2008, 2010a). Radiocarbon and luminescence ages support this interpretation and the correlation J I of the organic-rich units B and D with the last (MIS 2 to 4) and penultimate (MIS 6) glacials, respectively (Fig. 2). It must be acknowledged though that obtaining robust J I 20 and precise age control in arctic, partly reworked and weathered loess is very chal- Back Close lenging. Radiocarbon ages can be significantly too old, because soil organic material is exceptionally well preserved in cold, arctic environments, and re-working of sedi- Full Screen / Esc ments can occur through aeolian, fluvial or colluvial processes without mineralization of “old” carbon. On the other hand, the humic acid fraction, which for the above rea- Printer-friendly Version 25 sons is generally preferred over the refractory humin fraction when dating soils, is more Interactive Discussion susceptible to translocation, particularly in soils with waterlogging conditions. This can lead to too young radiocarbon ages. Similarly, the IRSL ages can be too old when 2202 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | the samples have been insufficiently bleached prior to deposition (which might explain the inconsistent IRSL age of 177 ka in unit C), or they can be too young due to signal CPD fading or saturation. For a full discussion of the available numeric ages, the reader is 6, 2199–2221, 2010 referred to the previous publications (Zech et al., 2010a). 5 In light of the dating uncertainties, the correlation of the organic-rich units with glacials remained controversial; particularly as palynological and biomarker results be- A permafrost glacial came available and documented
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