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Biogeochemistry

We didn’t start the fire!… that changed the southeast Australian landscape 44 thousand years ago

Article: Raquel A. Lopes dos Santos, Patrick De Deckker, Ellen C. Hopmans, John W. Magee, Anchelique Mets, Jaap S. Sinninghe Damsté, and Stefan Schouten. (2013): Abrupt vegetation change after the Late Quaternary megafaunal extinction in southeastern Australia. Nature Geoscience. Vol. 6, 627-631. DOI: 10.1038/NGEO1856

Background Information

Today, southeastern Australia is a lush temperate forest that is composed of at least 70% of plants which use the C3 carbon fixation pathway, most often attributed to broad-leafed trees.  However, this forest biome was very different 45 thousand years ago (kyr), during the Late Quaternary period. Instead, southeastern Australia was a huge grassland biome, similar to the African Savanna. So what caused this abrupt vegetation change?   Between 50 and 45 kyr ago in this same region, about 90% of the large animals (> 40 kg called megafauna) became extinct in an event called the Late Quaternary Megafauna Extinction. The cause of this extremely large and fast extinction has been a topic of interest to paleoceanographers and paleontologists because it could offer insights on how landscape changes and massive extinctions are related. It is always best to look into the past to make predictions for the future.   Prior to this study, there had been two competing hypotheses for the cause of this abrupt and sudden change in vegetation. The first hypothesis was that fire use by early human inhabitants caused the vegetation to change, resulting in less nutritious plants and this megafaunal extinction. The second hypothesis was that the extinction of these megafaunal grazers caused the grasslands to die off since the fire-prone plants were no longer being eaten.   This study used a marine sediment core to answer the question of who started the fires which changed the southeastern Australian landscape: humans or an animal extinction?

The Findings

Lopes dos Santo et al. (2013) used a sediment core recovered from the Murray Canyons Group region, which is located off the shore of Southern Australia (Fig. 1). This site is located very close to the mouth of the River Murray system. Marine sediments record what conditions were like in their regions by preserving biomarkers. Biomarkers are indicators for a biological process that can be used to reconstruct what ecosystems were like from hundreds to millions of years ago.

In this study, plant leaf waxes, called n-alkanes, were analyzed to assess what types of vegetation made up southeastern Australia and how/when that landscape changed during the Late Quaternary. These n-alkanes can depict vegetation species and changes with even higher resolution if their stable carbon isotope ratios (δ13C) are also measured. The Branched Isoprenoid Tetraether (BIT) index was used to suggest that the main source of the n-alkanes was from riverine transport. The BIT index is a proxy for river inputs of soil organic matter, so the authors could then hypothesize that the n-alkanes were from the local, southeastern Australia region and not from distant sources (carried by winds). Fire frequency was reconstructed with the biomarker levoglucosan, which is a combustion byproduct of carbohydrates specifically related to woody plants.

The n-alkane δ13C shows that grasses (which use C4 carbon fixation) were the dominant vegetation type during the warming between the glacial-interglacial event between 125-44 kyr. The grassland dominance was also associated with low fire levels (Fig. 2). The overall highest abundance of C4 grasses occurred between 58-44 kyr, which overlaps with the human “colonization” of Australia.   An abrupt change in vegetation from C4 grasses to C3 plants (such as broad-leaf trees) occurred between the narrow time-span of 44-42 kyr. There were no changes in the BIT index, suggesting that the input of terrestrial organic matter was the same during this event. Biomarker reconstructions suggest that C4 grasses went from composing >70% of the vegetation to ~38% during this time. These grasses were replaced by C3 vegetation that was much more prone to wildfires. This was shown in the sediment record as a huge spike in levoglucosan concentrations, which demonstrated that massive wildfires occurred after this abrupt vegetation change.

Fossil evidence shows that the massive megafauna extinction occurred around 46.4 kyr ago, which was before this vegetation shift! Thus, the extinction event is a probable cause for this shift in ecosystem vegetation. This finding suggests that humans were not responsible for these vegetation changes since the extinction happened before the wildfire peaks. Instead, it could be that the Late Quaternary Megafauna Extinction which caused the vegetation shifts.   Many of the megafaunal animals were herbivore grazers, or animals that feed on the plants. These grazers fed on the more fire-prone plants that grew in this area, preventing these shrubs and trees from ever becoming dominant. When these grazers were removed (by extinction), these fire-prone trees could grow freely and overtook the landscape.

To put this in modern day perspective, imagine a pasture filled with cows. Cows are constantly eating the grass and weeds, so pasture stays grassy. If we removed those cows, the grass in the pasture would become over grown with weeds, shrubs, and eventually trees would grow in this area (think of the lawn of an abandoned house). Without these Late Quaternary grazers, the grasslands in southeastern Australia became overgrown quickly and the temperate forests became the new landscape.   Of course, this vegetation change does not explain how and why those megafauna grazers became extinct. We now can hypothesize that it was not because of landscape changes and not because of human fire usage. Look forward to paleo-scientists investigating this further!

Significance

Human-controlled fire use was not the cause of the abrupt vegetation changes in southeastern Australia during the Late Quaternary. Rather, this paper hypothesizes that the massive extinction of large grazers caused fire prone C3 plants to become dominant over C4 grasses. The marine sediment record shows that these vegetation shifts occurred directly after the extinction event.   Thus, the modern day ecosystem of southeastern Australia was caused by the extinction of large grazers. Prior hypotheses had believed the opposite: that this extinction occurred because of these landscape changes. Scientists now are trying to determine the cause the Late Quaternary Megafauna Extinction, ruling out human-induced fires and poor dietary nutrition from a vegetation change.

Figure 1: Present day vegetation map of the study area. Colors indicate the modern day % of grasses were the darker green represents the lowest abundance of C4 grasses. The red dot (MD03-2607) is the location of the sediment core used in this study. The blue lines represent the River Murray. Black lines are summer/winter rainfall boundaries.

Figure 1: Present day vegetation map of the study area. Colors indicate the modern day % of grasses were the darker green represents the lowest abundance of C4 grasses. The red dot (MD03-2607) is the location of the sediment core used in this study. The blue lines represent the River Murray. Black lines are summer/winter rainfall boundaries.

 

Figure 2: The reconstruction of southeastern Australia’s vegetation using a marine sediment core between 60-30 thousand years ago. A) The water level of the lakes in the Wilandra chain. B) The reconstruction of sea surface temperature using biomarkers called alkenones. C) the n-alkane (plant leaf waxes) accumulation rates. D) the BIT index as a proxy for river-deposited soil organic matter. E) Levoglucosan accumulation rates used to assess fire events. F) Stable carbon isotope ratios of the n-alkanes to assess types of plants (C4 vs C3).

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