Present-day palynomorph deposits in an estuarine context: The case of the Loire Estuary

Review of Palaeobotany and Palynology

Vidal

Penaud

et al. 2017

Modern palynological investigations (pollen and non-pollen palynomorphs) have been carried out for the first time in the entire Bay of Brest (BB; NW France), a semi-enclosed oceanic basin flooded during the Holocene that today is exposed to both marine (North Atlantic surface waters) and fluvial (Aulne and Elorn rivers) influences. Palynological analyses were conducted on 41 surface sediments collected in several mudded bays where low-energy conditions prevail, allowing for fine particle decantation, and also on 12 moss samples considered to be pollen rain collectors taken on the periphery of the BB. Furthermore, the BB allows a direct comparison between landscapes and fossilized pollen assemblages in sediments thanks to relatively small watersheds and well-studied vegetation cover on land. Our data display a well-known distortion between pollen taxa percentages calculated from palynological slides and current vegetation cover on land, with a particular overestimation of arboreal taxa as well as an underestimation of herbaceous ones, especially of cereal crops that represent a major component of the regional continental vegetation. Various factors may be involved, including pollen transport issues, taphonomic processes, and different pollination rates according to trees/herbaceous taxa. Under close examination, we find that while all surface sediments show the same major species, palynological maps established in the BB regarding some specific pollen taxa exhibit differences and upstream-downstream gradients. Also, the comparison between the pollen content in BB surface sediments and in mosses supports the hydrodynamic influence acting on the pollen distribution in sediments. More specifically, we suggest that some arboreal taxa reflect a watershed effect, especially Alnus, which seems to be highly correlated to river influences and could therefore be considered as a potential proxy for fluvial Holocene palaeo-discharges within the BB.Highlights ►Modern palynological (pollen, dinocysts) investigations in a NW French estuary ► Distortion between pollen taxa percentages and vegetation cover estimated on land ► Overestimation (/underestimation) of tree (/herbaceous) taxa in marine sediments ► Alnus as a potential proxy for fluvial Holocene palaeodischarge

Section: Over-and Under-representation Of Arboreal and Herbaceous Polsupporting

confidence: 80%

“…Other tree pollen grains were frequently counted but never exceeded 20%, such as Castanea, Goff, 1997), as compared to other estuaries fed by major rivers and therefore draining large watersheds encompassing more varied environments (e.g. Loire watersheds in Ganne et al, 2016). A pie chart has thus been made to: i) summarize…”

Section: Pollen Datamentioning

confidence: 99%

Modern palynological record in the Bay of Brest (NW France): Signal calibration for palaeo-reconstructions

Review of Palaeobotany and Palynology

Vidal

Penaud

et al. 2017

“…2J). Lingulodinium machaerophorum, an estuarine-proliferating species 29,30 that tolerates strong drops in salinity 29,31 , testifies to coastal stratified waters subject to strong continental influence 11,21 and, thus, to stronger fluvial discharges at the studied site. Humid conditions together with an increase of fluvial inputs may explain the Corylus pollen rise during the 1.7-1.4 ka AP event in the BB.…”

Section: Resultsmentioning

confidence: 89%

Striking forest revival at the end of the Roman Period in north-western Europe

Penaud²,

Vidal³

et al. 2020

Sci Rep

Self Cite

The Holocene period (last 11,700 years BP) has been marked by significant climate variability over decadal to millennial timescales. The underlying mechanisms are still being debated, despite ocean–atmosphere–land connections put forward in many paleo-studies. Among the main drivers, involving a cluster of spectral signatures and shaping the climate of north-western Europe, are solar activity, the North Atlantic Oscillation (NAO) varying atmospheric regimes and North Atlantic oceanic gyre dynamics. Over the last 2500 years BP, paleo-environmental signals have been strongly affected by anthropogenic activities through deforestation and land use for crops, grazing, habitations, or access to resources. Palynological proxies (especially pollen grains and marine or freshwater microalgae) help to highlight such anthropogenic imprints over natural variability. Palynological analyses conducted in a macro-estuarine sedimentary environment of north-western France over the last 2500 years BP reveal a huge and atypical 300 year-long arboreal increase between 1700 and 1400 years BP (around 250 and 550 years AD) that we refer to as the ‘1.7–1.4 ka Arboreal Pollen rise event’ or ‘1.7–1.4 ka AP event’. Interestingly, the climatic 1700–1200 years BP interval coincides with evidence for the withdrawal of coastal societies in Brittany (NW France), in an unfavourable socio-economic context. We suggest that subpolar North Atlantic gyre strengthening and related increasing recurrence of storminess extremes may have affected long-term coastal anthropogenic trajectories resulting in a local collapse of coastal agrarian societies, partly forced by climatic degradation at the end of the Roman Period.

“…From 1965 onwards, it is notable that each recent peak of Alnus is associated with an even more pronounced drop of L. machaerophorum percentages, confirming the relationships between both fluvial and salt marsh signals, respectively. Despite its ecological coastal and estuarine preferences (Morzadec-Kerfourn, 1977;Ganne et al, 2016), L. machaerophorum is known to be very sensitive to nitrate and phosphate inputs, while these inputs can also benefit other species, such as S. bentorii (Zonneveld et al, 2013). A sharp increase of L. machaerophorum was observed in Norwegian fjords in response to the increased watershed eutrophication (Dale et al, 1999;Dale, 2009), while Pospelova et al (2002) showed that, in some American anthropized estuaries, L.…”

Section: Accepted Manuscript 24mentioning

confidence: 99%

“…over or under-representation) regarding the continental surface covered by vegetation and fossil pollen percentages calculated from modern sediment archives (e.g. Gaillard et al, 1998;Mazier et al, 2015;Ganne et al, 2016), as recently discussed for Bay of Brest watersheds versus modern sediments retrieved within the bay (Lambert et al, 2017). Furthermore, some studies highlight a large influence of climate-driven processes on the pollination rates of some trees (van Vliet et al, 2002;García-Mozo et al, 2006;Besancenot and Thibaudon, 2012), as well as on the transport of pollen grains through temporal variability of precipitation and thus river runoffs.…”

Section: Introductionmentioning

confidence: 99%

Human-induced river runoff overlapping natural climate variability over the last 150 years: Palynological evidence (Bay of Brest, NW France)

Global and Planetary Change

Penaud

Vidal

et al. 2018

For the first time a very high resolution palynological study (mean resolution of 1 to 5 years) was carried out over the last 150 years in a French estuarine environment (Bay of Brest; NW France), allowing direct comparison between the evolution of landscapes, surface water, and human practices on Bay of Brest watersheds, through continental (especially pollen grains) and marine (phytoplanktonic microalgae: cysts of dinoflagellates or dinocysts) microfossils. Thanks to the small size of the watersheds and the close proximity of the depositional environment to the mainland, the Bay of Brest represents an ideal case study for palynological investigations. Palynological data were then compared to published palaeo-genetic analyses conducted on the same core and to various available instrumental data, allowing us to better characterize past environmental variability since the second half of the 19th century in Western Brittany. We provide evidence of some clues of recent eutrophication and/or pollution that affected phytoplankton communities and which appears linked with increased runoff (higher precipitations, higher percentages of riparian forest pollen, decline of salt marsh-type indicators, and higher values of the XRF Ti/Ca signal), mainly explained by the evolution of agricultural practices since 1945 superimposed on the warming climate trend. We assume that the significant relay observed between dinocyst taxa: Lingulodinium machaerophorum and Spiniferites bentorii around 1965 then followed by Spiniferites membranaceus after 1985, attests to a strong and recent eutrophication of Bay of Brest surface waters induced by high river runoff combined with abnormally elevated air temperatures, especially obvious in the data from 1990. The structure of the dinocyst community has thus been deeply altered, accompanied by an unprecedented increase of Alexandrium minutum toxic form at the same period, as confirmed by the genetic quantification. Despite this recent major anthropogenic forcing, the fossil pollen sequence also records natural climate variability. We highlight, for the first time, a possible connection between climate (AMO modes) and fossil pollen records (especially tree pollination rates) in coastal sediments using tree percentage fluctuations as an indirect Highlights ► High resolution palynological study (last 150 yrs) in a French estuary ► Increased runoff on mainland since 1945 ► Increased eutrophication indices since 1980 ► Changes in phytoplankton communities and increased toxic algal blooms ► Connection between AMO modes and fossil pollen records (tree pollination rates)