Reflections on the Nitrogen Cycle and the Anthropocene workshop: Part 1

A big thanks to Melanie Leng, Carol Arrowsmith and other for putting the workshop on The Nitrogen Cycle and the Anthropocene together. It was a thought-provoking meeting and a great opportunity to both catch up with former colleagues and meet new faces.

I’ll start with a overview, largely for my own notekeeping on some of the points raised in the keynote talks, first quoting (copying and pasting) their abstracts where available, with the addition of a few links which I hope will be useful. This is part one of a two or three part entry.

Jan Zalasiewicz (University of Leicester): Stratigraphy of the Anthropocene: an overview.

“Human-driven rapid and large-scale change to the Earth system have led to the suggestion that we have left the Holocene to enter a new epoch of geological time: the Anthropocene Epoch. The term was proposed little more than a decade ago by Paul Crutzen, the Nobel Prize-winning atmospheric chemist, and has since been widely used – and sharply debated. Its status as a potential new unit of the Geological Time Scale needs evaluation by considering the various kinds of historical and environmental change in terms of geological – or more precisely stratigraphic – change. Lithostratigraphic change, for instance, is strikingly represented by the spread of ‘urban stratum’, the refashioning of sand, clay and limestone into your building, foundations and transport systems. Biostratigraphic change include the ongoing mass extinction event and the effect of invasive species (while deep human-made bioturbation in the form of extensive mine and borehole systems comprises a novel aspect in the fossil record). Chemostratigraphic changes include the reshaping of the Earth’s carbon, phosphorous and nitrogen cycles. Many of these transformations occur, though, at different times in different places. So, can an Anthropocene Series be effectively characterised and mapped across the Earth’s surface? Ongoing efforts to answer this question should help in the understanding of the Anthropocene as a new development within Earth history.”

Jan convenes the Anthropocene Working Group of the Subcommission on Quaternary Stratigraphy (International Commission on Stratigraphy).

Jan took us through a thought-provoking, nicely-illustrated and intriguing fact-filled (e.g. number of tonnes of plastic produced per year) presentation about the ways in which we might characterise the different stratigraphic components of a potential Anthropocene Epoch. The parts that particularly struck me were the ‘new minerals‘ being produced anthropogenically and how these metals, plastics and other compounds and how these are becoming, or will become, new rocks and new strata. In terms of the biostratigraphic signature he reminded us of the ‘homogocene’ term (e.g. Rosenzweig, 2001).

Jan encouraged us to question and ‘throw bricks’ at any of the ideas of how best to characterise and find the way to characterise the Anthropocene. His feeling was that nuclear signature might be the most distinctive and most easily traced stratigraphic tie-point between different types of depositional archive.

Questions about a potential tension between geological definition/identification and the emotive issues that the term invokes was raised.

Another idea was to consider the inter-planetary nature of the Anthropocene with footsteps on the moon and space vehicle debris…

This presentation reminded me of the BSG (British Society for Geomorphology) debate in 2013 at the annual meeting, held at Royal Holloway University of London, ‘Is there a geomorphological case for the Anthropocene’ (the link is a video of the debate, chaired by Stephen Tooth) and the importance the BSG community place on being involved in this debate and discussion. Tony Brown (one of the panellists) had led a 2013 paper in ESPL on this.

Within this, I should also flag up the paper by Jonathan Dean, Melanie Leng and Anson Mackay on isotope geochemical signatures for the Anthropocene. 

Tim Heaton (British Geological Survey): Sediment, soil and plant records of changes in 15N/14N ratios during the Anthropocene

“The annual production of reactive nitrogen compounds (nitrate, ammonium, etc) by human activities now exceeds production by natural processes; thereby more than doubling the amount of nitrogen available to the earth’s biosphere. If this ‘anthropogenic’ nitrogen finds its was into a lake then it may change the isotope compositions (15N/14N ratios) of the different components of the lacsutrine system. Crucially, because this anthropogenic nitrogen has mainly been produced in the past 100 years or so, it might be recorded in terms of a change in the 15N/14N ratios of the recently deposited sediment. Evidence for these changes is presented, and their significance discussed in terms of: differences between inhabited and remote environments; possible influences of diagenesis; information from terrestrial plants and the possible influence of other factors impacting on the nitrogen cycle.” 

Tim took us through the headline that since the 1980s the anthropogenic production rate of nitrogen compounds exceeds the natural production rates. Related to this point, I highlight the headlines that this has huge implications for the nitrogen cycle – in 2009 this made up section of a paper in Nature about the ‘safe operating space for humanity’. See a Yale BLOG post on this.

Tim’s focus was on lake sediments that record this anthropogenic nitrogen, showing us records from Lake Alexandrina in Australia and Lake Biwa in Japan, in which the increase in nitrogen is accompanied by a corresponding drop in the d15N. He also showed us other records from further afield, with examples from the Science paper by Holtgrieve et al. (2011) and remote lakes by Wolfe et al. (2013).

In summary Tim concluded that:

  • the decline in the d15N signature in lake sediments was real, and not entirely from diagensis in the sediment.
  • the favoured reasons for this decline were
    • the composition of atmospheric deposition (but there was poor, or a paucity of, evidence for anthropogenic N compounds have sufficiently lower d15N signatures
    • the amount of N deposition changing (increasing)
    • a potential link with atmospheric CO2 and the effect this has on soil de-nitrification.

Some useful links to work on diagnesis include a modelling study this year by Brahney et al. (2014).

If anyone has links to explanations about how the increased input of nitrogen (amount) compounds leads to an excess of inorganic nitrogen that Tim discussed, I’d be really interested to read more and try to get my head around this idea.

A reference relating to CO2 and N cycle with a focus on the oceans. Hutchins et al. (2009).

Thanks Lizzy for this recommendation of a paper discussing the links between global C, N and P cycles by Gruber et al. (2008).

Which is a good place to link Tim’s talk and emphasis on the last 100 years or so with the story of nitrogen cycling over the longer timescale of the Holocene (if this epoch isn’t subsumed, or made obsolete, by the Anthropocene). The changes in nitrogen cycling over the Holocene, recorded in a global-scale synthesis of lake records, by Lizzy and her colleagues Kendra, Joseph W and Joseph C (all at Kansas State University) can be found in Nature in 2013 (McLauchlan et al., 2013).




1 thought on “Reflections on the Nitrogen Cycle and the Anthropocene workshop: Part 1

  1. Lizzy

    Hi Abi, Great to see this! Here is a great paper for understanding the link between global C, N and P cycles: Gruber, N. & Galloway, J. N. (2008) An Earth-system perspective of the global nitrogen cycle. Nature, 451, 293-296.


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