Making Eden

by David Beerling

  rock weathering—that controls carbon dioxide levels in the atmosphere and cli-

  mate over millions of years. Trees were implicated as bioengineers of Devonian

  global change.

  The connections leading to this conceptual advance begin with the quest by

  roots, and their associated microbial partners, for nutrients, such as phosphorus and potassium, to support their growth. To mobilize these elements they deploy

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  a repertoire of metabolic tricks that acidify soils, chemically liberating the cargo of calcium and other elements from rocks and minerals. Indeed, the distinctive

  chemical signature of trees that Berner had detected in stream water in Iceland

  was a higher concentration of calcium and other elements compared to that in

  the stream water draining through his ‘naked Earth’ control catchment. These

  weathered products are transported to the sea by rivers where they react with carbon dioxide dissolved in seawater to slowly precipitate as solid calcium carbonate on the seafloor or become incorporated into the carbonate shells of organisms,

  such as corals, shellfish, and microscopic plankton called foraminifera. When

  they die, their shells sink in a continuous rain of carbonate debris that accumu-

  lates on the seabed. Slowly over millions of years, thick layers of carbonates build up on the floors of the world’s oceans, quietly sequestering carbon that was once in the atmosphere. The long-term sequestration of atmospheric carbon dioxide

  in this way weakens the greenhouse effect and slowly cools the planet. Eventually, hundreds of millions of years later, this carbon is recycled when plate tectonics folds the seafloor down into the crust. There the carbonates are subject to

  immense heating and compression on geological timescales of millions of years,

  converting them back into carbon dioxide gas, released by volcanoes into the

  atmosphere to continue its endless cycling around the planet.12

  Meanwhile, mindful of Berner’s successful scientific insights gained from

  working in Iceland, others later visited its basaltic terrain in the hope it would give up the secrets of how early land plants interacted with the planet’s rocky

  landscape millions of years before the trees took centre stage in the Devonian

  drama. The idea was to travel as far back in time as the Ordovician, over 450 million years ago, to the days when multicellular photosynthetic plants first arrived on land, and extract shallow rock cores, a few centimetres deep, beneath communities of bryophytes, fungi, and algae. Scrutinizing these mini rock cores with a suite of state-of-the-art geochemistry techniques produced little evidence for

  serious chemical alteration below depths of a few millimetres.13 At best, a few

  rock grains beneath liverworts showed features indicative of weathering, includ-

  ing tiny bowl-shaped pits and thin chemically altered weathering rinds, which

  were cautiously attributed to activities of the plants and their microbial part-

  ners.14 Based on this evidence, communities of simple land plants living in the

  early Palaeozoic failed to change the world.15 That development awaited the evo-

  lution of trees and the emergence of forests.

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  Or did it? Some scientists have speculated boldly that the first simple land

  plants were a ‘geological force of nature’ right from the off in the Ordovician.16 At that time, big landmasses of the world, such as Australia, Africa, and South

  America, were sutured together to form the supercontinent of Gondwana, situ-

  ated near the South Pole, and the atmosphere contained twenty times as much

  carbon dioxide as today. Yet there was a major glaciation in the Late Ordovician.

  Even after accounting for the reduced output from a younger Sun that burned less

  brightly in Ordovician times, it is difficult to trigger glaciations without drawing substantial amounts of carbon dioxide out of the atmosphere. Did scattered

  patches of early land plants clinging to rocks dramatically enhance weathering to pull enormous amounts of carbon dioxide out of the atmosphere and trigger glaciation? Did accelerated weathering on land repeatedly flush nutrients in shallow coastal waters to fuel algal blooms? If so, this might explain transient episodes in which burial of organic matter in the oceans increased, thought to reflect ancient pulses of marine productivity. Promising though it may seem at first glance, the

  flaw is that the shallow rhizoid-based anchorage systems of early land floras

  limited their zone of influence in the rocky landscape, as the studies in Iceland, and elsewhere, have shown.17 In some circumstances, lichens can actually protect rocks from the climate by smothering the surfaces and slowing down weathering.18

  Not surprisingly, the jury is still out on whether diminutive rootless floras drove the massive changes in global biogeochemical cycling necessary to trigger ice

  ages in Ordovician times.

  Contrast this with the situation that we think unfolded as large photosynthe-

  sizing trees and forests evolved and spread across the continents, marching into

  upland regions during the Devonian as the drive to capture sunlight took hold.

  Truly large plants with the arborescence habit had arrived. Massive tree trunks

  topped with stout branches adorned with leaves and leaf-like filaments emerged

  from a crowded forest floor (Plate 8). Trees went from reaching heights of a

  metre or so to over 30 metres within a few millions of years. The geological

  periods of the Devonian, and then the Carboniferous, saw the world populated

  with large trees as they stole a march on terrestrial floras from the tropics to the sub-arctic. Invoking the linkages discussed earlier, suggests that the rise and

  spread of trees forming the burgeoning forests locked Earth into a set of feed-

  backs along the pathway of cause and effect: bigger trees → deeper roots →

  more weathering → less carbon dioxide in the atmosphere → cooler planet.

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  It makes for a thrilling story, putting the evolution and spread of trees at the

  heart of a transformation of the Devonian world, but does this chain of cause and effect stand up to scrutiny? Support for two essential links in this scenario, dubbed the ‘Devonian plant hypothesis’, is written in fossil soils and sediments. Fossil soils point to a changing chemical composition of the atmosphere, indicative of a massive 90% decline in carbon dioxide levels19 that dramatically weakened the

  atmospheric greenhouse effect. Sediments show that the ultimate expression of

  this strengthening series of postulated Earth system feedbacks as a forested

  world took shape was the ending of the Palaeozoic greenhouse world. Out went

  the luxurious carbon dioxide-rich world of the early Palaeozoic. In came a new

  icehouse world that culminated around 300 million years ago in the Permo-

  Carboniferous glaciation, one of the most intense and prolonged series of severe

  glaciations in Earth history, with huge ice sheets extending from the poles to the tropics.20

  On these grounds, we might argue that Earth’s transition to a forested planet

  represents the most important natural biological feedback on climate ever

  witnessed over the past half billion years. We should be mindful, however, of the danger of this narrative being too simplistic, for there is a snag. It lacks the fossil evidence for early forest trees and their rooting systems engaged in weathering

  soil minerals in the way I have just outlined. For sure, intriguing pieces of evidence have turned up here and there21 from fossil soils and root traces, some

  of them not without the whiff of controversy. As Berner would have been t
he

  first to point out, the paradigm of cause and effect outlined above largely rests on weathering by contemporary forested ecosystems extrapolated back into

  the past. Nothing wrong with that, you might say—we have encountered this

  approach to understanding the history of life and the planet before. But what

  about interrogating the fossil record of early trees and soils to evaluate this

  crucial linkage directly?

  The quest for this missing link between the evolution of early forest trees and

  their geochemical engagement with soils and rocks brought us to a newly

  discovered fossil forest floor preserved in a quarry at Cairo, Greene County, New York State.22 The site is not far from Gilboa and the rocks and sediments there also date back to 385 million years ago. Stumps of a long extinct conifer-like progymnosperm tree related to Archaeopteris pockmark the floor, with spidery root traces radiating outwards across the dusty surface. Alongside these are many circular

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  depressions formed by giant cladoxylopsids. The quarry site at Cairo provided a

  unique opportunity—a locality with Devonian trees preserved in situ on the forest floor, and with tree rooting structures extending down into the sediments

  below that we hoped contained fossilized soil weathering profiles. Raising rock

  cores drilled from this ancient forest floor might, with luck, furnish vital clues for understanding how early trees and their rooting systems interacted with soils and soil minerals.

  Catching a geochemical glimpse of how these ancient trees interacted with the

  soils of the Devonian landscape was never going to be easy. Obtaining rock cores

  beneath the stumps of trees dotted across the quarry floor required a mobile,

  tall, truck-mounted drill-rig to solve the obvious engineering challenge (Plate 9).

  The business end of the corer on the rig was a diamond-encrusted cutter, cooled

  and lubricated with water drawn by pump from the nearby backwaters of the

  Schoharie Creek. Drilling and extracting the first rock core from beneath the

  main lateral root system of a larger archeopterid tree stump proved a tense

  affair. For those of us crowding round the rig, the anxiety was palpable. As the

  drill-rig lowered the corer and it touched down on the thin greenish-grey silt-

  stone capping the deeper sediments below, we had little idea what to expect.

  The compressed siltstone cap contains the remains of fossil fish that arrived on

  the fin after the forest was flooded. How deep was the layer of fossilized soil

  beneath the fishy siltstone cap? Would it shatter and split, destroying its geo-

  chemical story? Or would it retain its integrity and give us a shot at discovering what the roots of those early forests had been up to? As the drill penetrated the siltstone crust, coring the sediment beneath, brick-red water was flushed out of

  the drill-hole, painting the quarry floor bright red with 385-million-year-old ink made from fossil soils. A wave of relief and excitement broke out.23 Standing

  beneath the baking sun that afternoon, we gradually realized with mounting

  excitement that the drilling operations were likely to succeed as more-or-less

  continuous sections of sediment cores were raised to the surface. The drilling

  campaigns that followed extracted many cores extending 3 metres beneath the

  major early forest trees, providing raw materials for opening up detailed lines of investigation.

  The essential first step in the enterprise is to undertake painstakingly detailed sedimentary analyses down through each core, grain-by-grain, layer-by-layer,

  moving backwards in time as you go lower down the core. In this way, you slowly

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  build a minutely detailed log of changes in sediment texture, such as the size and shapes of the grains, colour, and so on. Compiling these sedimentary logs for

  every core meant we could then align the cores obtained next to the different tree stumps across the quarry floor by comparing common features in their profiles.24

  It became clear that the stumps sat on top of a sequence of fossil soils that said something about the Devonian climate the trees experienced and the soils in

  which they grew. Climate clues came from careful inspection of individual grains, which showed they had what soil scientists call ‘slickensided slip planes’. These distinctive features are typically only found in soils of warm sub-tropical regions experiencing seasonal rainfall, with the slickensided grains forming when soils

  contract on drying out and then expand on rewetting. Repeated expansion and

  contraction of soils causes tell-tale frictional wear of rock and mineral grains as they rub against each other. The Devonian trees, then, probably enjoyed a warm

  sub-tropical climate. Other features of the sediments suggested they were rooted

  in well-drained rather than waterlogged soils. Without this sort of evidence it is easy to become misled and Goldring herself thought the Gilboa trees had bases

  that were ‘bulbous, as might be expected of certain trees growing under swampy conditions’. The new findings revised old views suggesting that cladoxylopsids and

  Archaeopteris-type trees grew only on waterlogged, peaty soils.25

  More was to follow, because even casual scrutiny of the drill cores showed they

  displayed milky white root traces on the sides of cores (Plate 10). These markings, called ‘drab-halos’, are a record of the growth behaviour of the tree roots. A few had a central clay-rich cast running through them, preserving the structure of

  tree roots that once lived long ago. Mapping the depth of fossilized root traces in the rock cores showed the cladoxylopsid trees with simple roots were shallow and

  did not change much in depth with different tree sizes. For the conifer-like pro-

  gymnosperm trees in these early forest ecosystems, the story was strikingly dif-

  ferent, with larger trees having deeper rooting systems, and this supported a

  crucial missing link in the Devonian Plant Hypothesis. This sounds obvious

  until you remind yourself we are dealing with mysterious 385-million-year-old

  forest trees, and prior to this we knew next to nothing about the form and growth habit of these trees below ground.

  What about evidence for the next essential link in our chain of cause and effect, deeper roots → more weathering? Clues came from the numerous root traces preserved in the fossil soils with diffuse blue-grey haloes surrounding them, indicative

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  of chemically altered soils. Progressive changes in chemical composition down

  through the cores of fossil soils obtained beneath the conifer-like group showed

  that their roots had actively weathered rocks. Large trees caused much bigger

  changes in sediment chemistry than small trees. In other words the bigger the

  trees, the greater the intensity of tree-driven weathering.

  Support for this interpretation came from clay minerals. Clays are super-fine

  particles made by the weathering of silicate rocks—they even occur on Mars,

  formed after billions of years of chemical weathering, apparently in the absence

  of life.26 On Earth, though, clays form most strongly under forests because,

  strange as it may seem, trees are ‘clay mineral factories’.27 Trees intensify the weathering processes that erode rock into smaller and smaller particles, leading

  finally to clay formation. In fact, trees drive changes in the clay mineralogy of soils in a highly predictable direction, converting one suite of clay minerals into

  another in a characteristic manner over time, and this helps us interpret the past.28

  Since the 1960s, geologists had noted that Pre
cambrian sediments appeared to be

  composed of less weathered materials, with fewer clays than younger sediments.

  To explain this apparent conundrum (the expectation would be that older sedi-

  ments had more clays), they speculatively attributed the increased abundance of

  certain clays (smectite and kaolinite) in soils through the Silurian, Devonian, and Carboniferous to the actions of an expanding forested biosphere.29 The analysis of the rock cores from Cairo suggested increased production of clays with the effects appearing to be greatest beneath the largest trees with the deepest rooting systems.

  When put together with the other geochemical evidence, it starts to build support for the chain of cause and effect: bigger trees → deeper roots → more weathering.

  Fascinating though these studies are in helping us piece together the case for

  trees changing the world, they still do not yet tell the whole story. The past decade has witnessed the emergence of a new, more inclusive picture, recognizing symbiotic soil fungi as long-overlooked players in the biogeochemical cycling of

  elements.30 Berner, of course, was on to this possibility straight away. He had

  already collected samples of Hawaiian basalt from beneath rooted plants and

  observed a porosity absent from the same basalt flows uncolonized by plants.31 The explanation, he hypothesized, was ‘solutions secreted by symbiotic microbiota

  associated with plant roots’. Under the microscope, a slide containing a cross-

  section of basalt rock showed a channel formed by a thread of a mycorrhizal fun-

  gus connected to a tree chemically dissolving its way through to a pocket of

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  nutritious minerals. Had the fungus mined and supplied these to its host plant by chemically tunnelling through the rock? Others had already been thinking along

  these lines,32 suggesting that the highly localized acidification occurring at the physiologically active tip of a fungal thread created a reactive front line that chemically etched out ‘micro-burrows’. Alternatively, Berner’s fungal thread may have simply grown through a pre-existing tunnel. Some tunnels are the work of chemistry without the intervention of biology and tiny holes and tunnels can develop as chemical reactions dissolve minerals preferentially along structural planes in the packing of the crystal lattices.33