Monitoring fluvial pollen transport, its relationship to catchment vegetation and implications for palaeoenvironmental studies

Abstract

Despite being the most important source of pollen and spore input into most lakes and near-shore marine sediments, we know very little about fluvial (waterborne) pollen and spore transport. This paper presents the results of a dedicated monitoring programme conducted over 2 years and at a catchment scale in South West England. The land use of the nine sub-catchments monitored was determined using Landsat Thematic Data. At two stations, pollen and spore sampling through storm hydrographs was undertaken whilst at the other 7 sub-catchments only peak flow samples were collected. Samples were also collected from re-suspended bed material, riverbanks and at low flows. Airborne pollen flux was monitored using modified Tauber traps. The results support previous research illustrating how the vast majority of fluvial pollen and spores are transported during floods (in this case 91%) and that the main control on waterborne pollen and spore assemblages is the catchment vegetation. However, strong seasonal effects are shown as well as the importance of distinctive sources, such as the riparian input, bed re-suspension and overland flow into drains and tributaries. Fine sediment in river pools appears to act as a selective store of damaged cereal-type pollen grains in arable catchments and this can reduce the inherent underestimate of arable land from pollen diagrams with a high fluvial input and increase the visibility of early agriculture. In order to simulate the likely result of a flood-dominated influx to a small lake scenario, modelling was undertaken whereby different sub-catchments were substituted in order to represent changes in catchment vegetation under a constant hydrological regime. The results show the dampened response of land use groups to catchment land use change, and the frequent occurrence of anomalous single-level peaks due to seasonal flushes from specific near-stream vegetation types. Both these features are commonly seen in lake pollen diagrams. Fluvial pollen and spore loading is dependant upon discharge and so concentrations in laminated or varved sediments could be regarded as a proxy for flood magnitude. The implications for this study on the interpretation of lake and near-shore marine pollen and spore diagrams are discussed and it is argued that a more quantitative approach to waterborne pollen could improve the estimation of land use from lakes in the temperate zone.

Pollen source areas of lakes with inflowing rivers: modern pollen influx data from Lake Baiyangdian, China

“…Most lakes have inflowing rivers, so waterborne pollen may also make a substantial contribution to the pollen influx to the lake (McAndrews and Power, 1973;Peck, 1973Peck, , 1974Bonny, 1978;Sun and Wu, 1987;Traverse, 1992;Huang et al, 2004;Xu et al, 2005;Brown et al, 2007). In order to attempt quantitative reconstructions of past vegetation and paleoclimate from the sedimentary record in such lakes, it is important to understand the relative importance of the different components of the pollen taphonomy and to characterize the spatial sensitivity of the record, the pollen source area.…”

“…Brush and Brush, 1972;Fall, 1981;Solomon et al, 1982). Flume experiments suggest that pollen grains will settle out into sediment when the water speed is lower than 0.3 m/s, and therefore that grains can remain in suspension and be transported long distance when the water speed is higher than 0.3 m/s (Brush and Brush, 1972).…”

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“…The mugworts (Artemisia-type), members of the carrot family (Apiaceae), cabbage family (Brassicaceae), goosefoot family (Chenopodiaceae) and other docks and sorrels (Rumex acetosa/acetosella) are common by rivers and on disturbed ground (Brown et al 2007).…”

Palynomorph Preservation in Volcaniclastic Rocks of the Miocene Tepoztlan Formation (Central Mexico) and Implications for Paleoenvironmental Reconstruction

“…Therefore, the condition of preserved pollen can yield insight not only into differential preservation in a system but possibly also into the transport history of the pollen grains (c.f., Campbell, 1999). The ratio of fluvial to airborne (both wet and dry) transport and input depends on the relationship between the size of the catchment, the topography and the catchment vegetation (Brown et al, 2007).…”

“…Fluvial transport is most important for spore and pollen dispersal (Streel and Bless, 1980) and later burial in sediment, with transport distances up to several hundred kilometers known from recent river systems (Chmura et al, 1999). The fluvial pollen and spore load itself is a combination of several distinct components including: 1) an airborne component (directly into the channels from local to regional sources); 2) an overland flow component; 3) a bank erosion component; and 4) river storage in bed-sediments that can be resuspended during floods (Jacobson and Bradshaw, 1981;Brown et al, 2007). The same applies for debris flows and hyperconcentrated flows in volcanic environments (lahars) where palynomorphs are either introduced by wind, overland flow of water or erosion of loose volcaniclastic sediment.…”