Pine pollen clues reveal a timeline of sedimentation at Aldinga Washpool

In our most recent article on the Aldinga Washpool, Adelaide’s only remaining coastal lagoon, we mentioned our ongoing investigation into sedimentation on the original bed of the lagoon. We know that since European colonisation, a lot of sediment has been deposited into the Washpool, making it shallower. This sedimentation is evident from Kaurna knowledge, early colonial accounts, previous studies and the deep erosion gullies that are present in the catchment.

As part of the Green Adelaide funded Grassroots Grant we have been working on since September 2023, we have collaborated with Associate Professor John Tibby and colleagues at the University of Adelaide to investigate the thickness of this layer of post-colonisation sediments sitting on the original bed of the lagoon. We’re especially interested in the area within the proposed excavation footprint presented in our 2022 report. We’ve taken soil cores at 21 locations (see below), mostly within the proposed excavation footprint, but also outside of it for comparison. The material in these cores has been used to answer other questions, which we have reported on in previous articles, but in this article we focus on the thickness of the recent sedimentation.

When each core was extracted from the ground John and his team took sub-samples from different depths down the core profile. These subsamples were taken back to the lab and processed to concentrate any pollen grains of Pinus radiata into a small volume of solution that was then examined under the microscope. By detecting Pinus pollen in sediments we can determine how recently they were deposited. Pinus radiata was introduced to South Australia in 1876 and by 1900 was being planted widely as a plantation timber. It’s pollen disburses widely in the air, generally preserves well in sediments and is relatively easy to identify from the pollen of other species. Pinus pollen detected at depth in a sediment core is an unambiguous indicator that all the sediment above that depth was deposited after about 1900, i.e. post-colonisation. The figure below shows the maximum depth below ground level (in cm BGL (below ground level)) that John’s team detected Pinus pollen in our sediment cores at the Washpool.

Some points to note about these numbers:

Firstly, most cores had Pinus pollen below the depth to which the excavation would need to occur. While the excavation depth is yet to be determined at a fine scale, it would certainly be no deeper than the adjoining deeper areas of the lagoon, and most cores have Pinus deeper than that level. However, there are a few cores where the deepest Pinus detected was above the level we might want to excavate down to. This raises a dilemma: should we excavate less deeply in those areas, or are there other factors to consider?

It’s important to note that the Pinus pollen method isn’t perfect, and it is possible to miss Pinus that is actually present. It’s also possible that Pinus pollen hasn’t always preserved well in the Washpool sediments. As John puts it, “absence of evidence is not evidence of absence”. In other words, the maximum depth at which we detected Pinus does not mean that the sediment immediately below that depth is the original, pre-colonisation bed of the Washpool. In fact, we strongly doubt that this is the case because other key characteristics of the sediment, i.e. its colour and texture, were typically the same above and below the maximum Pinus depth and also the same between cores. That was until we got very deep. Two of the cores we took, Core 20 and Core 21, were obtained with a mechanical auger that penetrated much deeper than the other cores, down to 300 and 200 cm respectively. These cores showed a distinct change at 155 and 129 cm respectively, where the homogeneous grey-brown clay above (that was similar to all other cores) changed to a much paler clay soil horizon (see image below), possibly indicating we had penetrated into the pre-colonisation bed of the Washpool.

Another point to consider is that detection of Pinus at a deep level in a sediment core can provide confidence about the depth of post-colonisation sedimentation in its general vicinity that might over-rule data from other cores. Core 20 is a case in point. At this core Pinus was detected at 115 cm BGL, considerably deeper than nearby cores. This provides further evidence that the homogeneous grey-brown clay below the maximum Pinus depth in nearby cores was very likely deposited after colonisation.

The third point to make is that even outside of the proposed excavation footprint Pinus was detected well below the surface, e.g. at 49 cm BGL at core 06. This tells us that the area of the Washpool affected by sedimentation is likely larger than the proposed excavation footprint and that the excavation proposed is relatively conservative. This is important because we need to leave a buffer of recent sediments sitting over the original bed of the Washpool to protect cultural heritage.

This study provides a high level of confidence that the proposed excavation to (partly) restore the original shape and volume of the Washpool lagoon would only remove sediment that has been deposited after European colonisation. Across much of the proposed excavation footprint we have the “smoking gun” evidence of Pinus pollen below the proposed excavation depth. In other, smaller areas we need to extrapolate to some degree but the similarity of sediments with those containing Pinus provides confidence they are part of the same, post-colonisation sediment horizon.