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).

 

A WRITE UP OF JAN KAISER and ERIC WOLFF’s KEYNOTES and some reflections on the DRYLAND STORY IN THE KALAHARI WILL FOLLOW IN PART 2. (to be completed…)

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The Nitrogen Cycle and the Anthropocene

Next week (Wednesday 29th October) the British Geological Survey at Keyworth are hosting a workshop on The Nitrogen cycle and the Anthropocene.

From the advertisement flier for the event:

“The rationale for this is that there are several types of temporal record (ice cores, sediments, tree rings) which show a reduction in 15N/14N ratios during the ‘Anthropocene’, a period in which there has been a substantial increase in the amount of reactive nitrogen in the earth’s nitrogen cycle. These changes are thought to be mainly due to the industrial synthesis and application of fertilizers, other changes in farming, and the combustion of fossil fuels. However, there does not seem to be any general agreement on the mechanism/s which cause this change in 15N/14N ratios, or indeed whether the changes in different records are related.

The workshop will therefore aim to promote discussion around: 15N depletion in organic matter in recent lake sediments; the lag between 15N in recent ice cores and lake sediments; recent 15N changes in modern plants/trees and soils; changing sources of N in glaciers and ice cores; and on how changes in source inputs to the atmosphere, and/or changes in its chemistry processes during the past few hundred years might have resulted in a decrease in 15N/14N of deposited N?”

There will be both keynote talks during the day and posters on display. The keynote talks feeature a stellar line-up of Earth Scientists.

• Jan Zalasiewicz (University of Leicester): Stratigraphy of the Anthropocene: an overview.
• Jan Kaiser (University of East Anglia): Isotopic evidence of sources and chemical processing of nitrogen in the atmosphere.
• Eric Wolff (University of Cambridge): Ice core signals of a changing nitrogen cycle.
• Tim Heaton (British Geological Survey): Sediment, soil and plant records of changes in 15N/14N ratios during the Anthropocene.

I will be presenting a poster Naturally-high nitrate in unsaturated zone sand dunes above the Stampriet Basin, Namibia which is the output of a paper in the Journal of Arid Environments with Mike Edmunds, that shows: (i) high natural nitrate production in the unsaturated zone sediments above the Stampriet Basin. most likely from vegetation; (ii) that this high nitrate is variable across space; (iii) that nitrate is being introduced as pulses and moving toward groundwater and (iv) this finding is in line with observed nitrate concentrations in other drylands. Elevated nitrate is a difficult water quality issue to manage in dryland environments.

 

 

(Practising) communicating geoscience (Posters and Pritt Sticks?)

I am presenting a poster at the IAG (8th IAG International Conference on Geomorphology – August 27th to 31st, 2013) in Paris next week. Eating lunch amongst humanities colleagues this week, the idea of a poster at a conference raised some chuckles and offers of Pritt Stick. This perhaps highlights some of the challenges of communicating how research is communicated in my field of geoscience (and in many other science disciplines) to other academics. So, where does this leave the much thornier issue of communicating science to the public?

Jon Tennant, who blogs at an EGU (European Geophysical Union) hosted blog GREEN TEA AND VELOCIRAPTORS, wrote (back in Oct 2012) about communicating research and knowledge to a wider public. This was orientated around ‘why bother’ and ‘what might the public audience already know’, which very effectively sets up an idea both of the motivation and the context to any endeavour to communicate geoscience research to the public (probably largely through blogging). Jon’s work was itself prompted by work by Prof Ian Steward and Ted Nield.

I have previously produced 1-page handouts (snippet in the picture) for land-owners at my sampling sites to give an idea of why I spent a few days digging holes into the sand in the heat of the desert.

So. The research I’m presenting next week, quite honestly didn’t start out as much. No pressing question set up in an academic paper, no large grant was awarded, rather it was an interesting view driving through the stunning Namibian landscape on the way to (another) conference, hosted at the Gobabeb Research Centre. After miles of mountainous terrane, and a descent down the hair-pin bends of the Gamsberg Pass, a very familiar colour of sediment to me was seen at the side of the road – red sand dunes (I’ve spent plenty of time digging holes into red sand dunes, sporting terrible fashion sense, both in the Kalahari, and the Namib Sand Sea, to the top of the latter we were travelling).

Drilling holes in sand dunes 

I asked my travelling companion, a Namibian Geographer, if he knew where that beautiful red sand had come from, and he didn’t have an answer. Stopping for a cool drink at the lodge named after the red sands (Rooisand) it seemed they also weren’t completely sure, but that it was suggested the sand had blown over and down the mountain pass we had driven down, having starting off in the Kalahari, famous for it’s long, red, linear sand dunes. Once at the conference, I still couldn’t find the definitive answer I was looking for, with someone else suggesting the sand might have blown up north-east from the Namib Sand Sea (which would make it the most northerly part of the sand sea). So, on the way home, armed with some cutlery and sampling tubes, I decided to collect a bit of this sand for analysis from an exposure at the side of a dry river bed.

The exposure where samples were taken

… At this point it is worth asking you (the reader) whether asking ‘where the sand has come from?’ is an interesting enough question worthy of some geoscience research and effort? Maybe it is. However, much of my research field of Quaternary Science (go on, Google/ Wikipedia it) is about asking what piles of sediment (and other deposits like ice) can reveal to us about what the climate of the past was like. In this light, the question becomes when and by what process (wind deposition, water-deposition followed by wind deposition) did this loose sand accumulate here, and what was the climate like in order for this to occur? Today, the area is very dry, and only seasonally windy enough to move some of it around. Was this different in the past?

Rooisand from the air (via GoogleEarth) 

So, now I had my sample, how was I going to settle the question of where this red sand came from? The term that’s used in geoscience for this is ‘provenance’. In addition, what might I be able to say about how long the red sand had been sitting there in the landscape for folks like me to catch a glimpse of as we travelled past on our way to the Namib Sand Sea or the coast (or how old the Rooisand deposit is)? A cool technique to establish this, is known as luminescence dating (making sand glow to establish how long it has been buried under other piles of sand, but that will have to be the topic of a separate blog post). My poster for next week is therefore imaginatively entitled ‘Age and provenance of the dunes of Rooisand’.

So, here beings my communication challenge.  What has been done with those samples to answer my questions? (my methods). And, what do the results of these tell me, and my coauthors, that the answer is?

And a reminder of the possible answers about Rooisand provenance: from the Kalahari, from the Namib Sand Sea, or an alternative idea that it is very locally sourced (material eroded from the surrounding mountains).

When you take a handful, or even a fingerful, of the red sand and put it under a microscope it isn’t all red sand at all. Some of it is white, some milky-white, some translucent, some green, some shiny black, etc. etc. (I’ll try to source a picture, but it’s not quite as cool as the ocean sand pictures that have been doing the Twitter rounds, largely because it is not as full of old marine life). This microscope vision is not only pretty, but pretty useful for working out where the sand came from (provenance). In Italy, after some preparation, including putting grains onto slides, (to make it easier to tell exactly what these sand grains of different appearance are) Mara Limonta and Giovanni Vezzoli at the University di Milano-Bicocca counted and characterised a few hundred grains. They had been doing this for lots of other piles of sand collected from other sites in Namibia, some from river valleys, some from Kalahari, some from the Namib Sand Sea.

What all of this microscope work revealed is that samples from different locations contained different amounts of various minerals and heavy minerals (by this I mean an array of terms, some of which may be more familiar sounding than others – quartz, K-feldspar, plagioclase, muscovite, biotite, epidote, amphibole, zircon, rutile, tourmaline). Ultimately, the details of what the minerals are is not crucial to the story. What is, is that the hard-rocks at the surface of the earth (what we could call the bedrock geology, or consolidated hard rock material, as opposed to the soft sands at Rooisand) in different places contain different combinations and proportions of these minerals. In effect, the sediment (whether rock or loose sand) has a fingerprint. So, once a rock has been broken down by weathering into smaller grains of sand, and redeposited somewhere in the landscape (like at Rooisand), it is possible to use it’s fingerprint to work out where it came from. Similarly, the difference between sand sample fingerprints (from different river channels and areas of sand dunes) can be instructive.

It was a diagram plotting the difference between these sands that arrive in my inbox from Italy last month for presenting on the poster.

The plot from Italian colleagues

In this plot the squares represent river sands, the stars wind-blown dune material, and the straight coloured lines are a sneaky way of plotting lots and lots of information about what minerals are in the samples without being constrained to two simple axes:  x (horizontal) and y (vertical) axis. In addition, the closer a box or star is to a line the more of that mineral (the coloured line) that sample contains. In addition, the closer the boxes and stars are to other boxes and stars, the more similar their fingerprint is (and therefore, they are very likely to have come from the same place and same bit of hard-rock geology). So, in short that plot shows me that the Rooisand sample (RS12/1) is most similar to samples from Rivers Gaub, and Kuiseb and closest to the coloured lines that represent heavy minerals such as garnet (and also apatite and amphibole) (all in the top left-hand box). The sample is very different to bottom right-hand box, where the sand samples from the Kalahari are plotted (so, I’m afraid I will have to tell the barman that served our cool drinks that the idea they have at the Rooisand lodge is wrong).

So, if the Rooisands are most similar to Kuiseb River sands and contain garnet, apatite and amphibole, what does this say about their provenance? Those suites of minerals are derived from a geological unit known as Damara metasediments (metasediments are sediments that some time after deposition, become subjected to heat and pressure, with changes to their properties, as mountains, like those in this part of the Namibian landscape are formed). These Damara metasediments form a band that runs through the Rooisand area, and so are very local.

To be able to answer the question about the age of (the last) sample burial event, I am awaiting one last piece of data (about something called dose rate, again this will have to be the subject of that other post on luminescence dating). What I can currently estimate is that this exposure close to the river bed is only a few hundred years old. It was deposited on that occasion by the wind. The average size of the grains is just over 300 microns (that’s 0.3 of one of those millimetre lines depicted on rulers), and this requires wind speeds of over 8 meters per second to move the sand. If all 10 meters of the exposure were deposited around a similar time in the past, then it would suggest that winds were over 8 meters per second sufficiently often during those years to move and then put back down 10 m of red sand. That’s a windier environment than today.

So here ends my first rehearsal of science communication, and as well as being slightly too long, it has revealed to me a more pressing question, why are the Rooisands, red sands (and not any other colour)? After all, that is what about them that caught my eye in the first place, and made the Rooisand lodge suggest the sand had travelled from the Kalahari. In short, they are coatings of iron oxides (just like rust). This rusting occurs in dry environments when the sediment and iron minerals are directly exposed to the atmosphere. The complicating factor is that it is possible the red coatings on these grain could have been inherited from the much-older geological hard-rock, or have accumulated since the soft sand sediments were deposited. This geoscience question currently remains unanswered.