One of the problems with writing a book that tries to describe the current state of any science is that, by the time the book is published, the current state will have moved on and there’s always something new and wonderful in that science. And so it is with The Desert.
As we struggle to come to grips with the startling roles of our planet’s arid lands in the way the earth system works – landscapes, surface processes, atmosphere, oceans, climate and ecosystems – the more we appreciate how much catching up we have to do. Our understanding of arid lands lags significantly behind that of their more accessible and pleasant temperate equivalents. The shear scale of deserts and desert processes makes comprehensive data collection and integration a serious challenge – it’s not that long ago that Ralph Bagnold, ‘the man who figured out how deserts work’ had to give up on his pioneering analysis of wind-blown sand simply because the data on winds in the desert did not exist.
Dust is a great player in earth system games and the Sahara is the greatest source of natural dust. Yet the role of dust is only now being understood, and it is full of surprises, some good, some less so. This key topic is something that I try to reflect on in the final chapter of the book, looking at the challenges and the future of arid lands, but the science moves on. It has long been known that Saharan dust travels across the Atlantic and plays a role in the rainforests of the Amazon and the ecosystems of the tropics. I was intrigued to read that Saharan dust is responsible for soil development – and therefore agriculture - in the Caribbean region, and duly reported this startling fact. But now it seems very possible that African dust has been, and continues to be, responsible for the building of the Bahamas and biological activity in that otherwise nutrient-poor region.
The Great Bahama Bank is great indeed – over a hundred thousand square kilometres of carbonate sediments that have accumulated over at least the last 100 million years to build up a thickness of several kilometres. And it’s essentially all mud. Not shell fragments, not coral debris, mud.
To say that this has long posed a conundrum for geologists is something of an understatement. Arguments and hypotheses have swirled around the academic world like the sediment and atmospheric patterns around the bank itself, but now a research team from the Universities of Miami and Amsterdam have proposed a radical mechanism for the build-up of the Great Bahama Bank – Saharan dust. From the University of Miami press release last week:
MIAMI – A new study suggests that Saharan dust played a major role in the formation of the Bahamas islands. Researchers from the University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science showed that iron-rich Saharan dust provides the nutrients necessary for specialized bacteria to produce the island chain’s carbonate-based foundation.
UM Rosenstiel School Lewis G. Weeks Professor Peter Swart and colleagues analyzed the concentrations of two trace elements characteristic of atmospheric dust – iron and manganese – in 270 seafloor samples collected along the Great Bahama Bank over a three-year period. The team found that the highest concentrations of these trace elements occurred to the west of Andros Island, an area which has the largest concentration of whitings, white sediment-laden bodies of water produced by photosynthetic cyanobacteria.
“Cyanobacteria need 10 times more iron than other photosynthesizers because they fix atmospheric nitrogen,” said Swart, lead author of the study. “This process draws down the carbon dioxide and induces the precipitation of calcium carbonate, thus causing the whiting. The signature of atmospheric nitrogen, its isotopic ratio is left in the sediments.”
Swart’s team suggests that high concentrations of iron-rich dust blown across the Atlantic Ocean from the Sahara is responsible for the existence of the Great Bahama Bank, which has been built up over the last 100 million years from sedimentation of calcium carbonate. The dust particles blown into the Bahamas’ waters and directly onto the islands provide the nutrients necessary to fuel cyanobacteria blooms, which in turn, produce carbonate whitings in the surrounding waters.
Persistent winds across Africa’s 3.5-million square mile Sahara Desert lifts mineral-rich sand into the atmosphere where it travels the nearly 5,000-mile northwest journey towards the U.S. and Caribbean. The paper, titled “The fertilization of the Bahamas by Saharan dust: A trigger for carbonate precipitation?” was published in the early online edition of the journal Geology.
The conclusions of the paper are as follows (don’t, as I was initially, be confused by the term ‘whiting’ – it’s not the fish, but, as explained above, ‘white sediment-laden bodies of water produced by photosynthetic cyanobacteria’):
In this paper, we demonstrated a strong similarity between the distribution of whitings and the Fe in the sediments of the GBB. We believe that the Fe originates from dust deposited either directly or washed in from dust deposited on the adjacent Andros Island. The input of Fe helps induce the precipitation of CaCO3 through the photosynthetic activity of cyanobacteria. Cyanobacteria also fix N2, which is utilized by all the biological communities on the GBB and is evident in the d15N signature, which is close to zero over the entire platform. Such whitings might be responsible for helping to produce vast amounts of sediments, not only within recent times, but also during previous periods of geological history. Evidence of long-term dust deposition is present in other records and has been postulated to account for the accumulation of soils through the Caribbean and the southern United States (Muhs et al., 2007). Such production might be significantly increased during periods of high dust input and could account for variations in rates of accumulation and platform progradation. This model suggests a modification to the paradigm that proposes that high concentrations of nutrients are detrimental to the growth of carbonate platforms. Rather we propose that certain nutrients may promote platform growth, particularly ones dominated by nonskeletal carbonates and the formation of mud by whitings.
This is fascinating stuff and of potential significance way beyond the Bahamas and the present time – as the paper notes:
This phenomenon might be responsible for the formation of vast amounts of sediments in the oceans, not only within recent times, but throughout geological history, particularly in the early history of the Earth prior to the existence of calcium carbonate–secreting organisms.
This didn’t make it into the book, but then again, that’s what this blog is for.
[Thanks to Suvrat Kher at Rapid Uplift for bringing this to my attention. Images courtesy of the NASA Goddard Space Flight Center. The original paper: P. K. Swart, A. M. Oehlert, G. J. Mackenzie, G. P. Eberli, J. J. G. Reijmer. The fertilization of the Bahamas by Saharan dust: A trigger for carbonate precipitation? Geology, 2014; DOI: 10.1130/G35744.1]