Impacts of late Holocene rapid climate changes as recorded in a macrotidal coastal setting (Mont-Saint-Michel Bay, France)
- 1Morphodynamique Continentale et Côtière, Université de Caen, UMR CNRS 6143, 24 rue des Tilleuls, 14000 Caen, France
Abstract
The impacts of rapid climate changes during the Holocene are well documented in deep oceanic and lacustrine sediments. Until now, no studies have shown the effects of rapid climate change on tidal successions in coastal wedges formed during the late Holocene transgression. Cores and very high resolution seismic data collected in Mont-Saint-Michel Bay, France, a macrotidal setting, demonstrate that rapid climate changes, with ~1500 year periodicity, are recorded in the sedimentary successions that constitute the late Holocene infill of the bay. The sedimentary expressions of rapid climate changes vary according to the different subenvironments within Mont-Saint-Michel Bay; cycles, a few meters thick, can be correlated throughout the bay, and radiocarbon dating suggests that they have a millennial time scale. The various changes reflect an increase in wave dynamics in association with Bond cold events, possibly in conjunction with long-term (1800 year periodicity) tidal cycles.
INTRODUCTION
In the context of present-day global warming, many studies are being carried out to better define the environmental impacts of rapid climate changes. High-amplitude climatic instabilities during the postglacial pre-Holocene period (before 10 ka ago) have been recognized for many years. Paleoclimate records have shown that Holocene climate was punctuated by widespread cooling events, recurring every ~1500 ± 500 years (Bond et al., 1997; Bianchi and McCave, 1999; Broecker, 2000; Mayewski et al., 2004; Debret et al., 2007; Allen et al., 2007). The response of coastal areas to rapid climate changes could be assumed to be rapid, with limited inertia, as their behavior is related to climatic agents, such as storms. However, the difficulty in studying rapid climate change impacts on coast behavior during the Holocene is to find complete records, as coastal settings are characterized by frequent high-energy events that destroy parts of the sedimentary record.
Direct impacts of rapid climate changes on coastal sedimentary successions are poorly documented. Generally, changing pollen or fauna content in marine or estuarine sediments remains the main proxy to reveal the occurrence of Holocene rapid climate changes (e.g., de Menocal et al., 2000; Desprat et al., 2003; Willard et al., 2005; Leorri et al., 2006). Zazo et al. (2007), by using different approaches on various data from various sources (e.g., marine cores, cliff cross sections), demonstrated the impacts of cyclic climate (and/or eustatic) changes on the postglacial evolution of the southern coasts of Spain. Cyclical changes in barrier progradation rate were directly related to millennial-scale climatic variations, possibly correlated to late Holocene Bond cold events (Bond et al., 1997).
Studies based on direct sedimentological data and very high resolution sequence stratigraphy that reveal coastal landscape modification under the impact of Holocene rapid climate changes remain rare (e.g., Sorrel et al., 2009). Here we report evidence of millennial-scale rapid climate changes in a late Holocene coastal wedge in a macrotidal setting, Mont-Saint-Michel Bay (France). Our objective is to show that evidence of rapid climate change cycles can be preserved in a high-energy coastal setting.
STUDY AREA AND AVAILABLE DATA
The Mont-Saint-Michel Bay, located along the southern coast of the English Channel (Fig. 1), is one of the best known macrotidal environments in the world. The tidal range is as high as 15 m during equinoctial spring tides. Mont-Saint-Michel Bay is a wide and shallow depression incised into a Precambrian substratum. Only the last postglacial transgression is recorded in the sedimentary infilling of the bay because the previous sea-level fall was accompanied by fluvial erosion to bedrock (Larsonneur, 1994). As a consequence, the coastal sediment wedge of the bay is almost entirely Holocene, with only few remnants of Pleistocene fluvial terraces.
Location and bathymetric map, Mont-Saint-Michel Bay.
The sea reached almost the present-day coastline ca. 8 ka ago. The global sea-level rise that followed the last glacial maximum was rapid (~6 mm/a; Lambeck, 1997). By 7–6 ka ago, the sea-level rise had started to slow (3 mm/a to 1 mm/a; Lambeck, 1997; L'Homer et al., 2002). This resulted in a rapid infilling of the bay by offshore-derived sediments that were reworked landward by flood-dominated tidal currents. Fluvial sediment supply was negligible. Since that time of slow transgression and highstand conditions, Mont-Saint-Michel Bay has progressively acquired its present-day composite landscape. According to the balance of tide, wave, and fluvial dynamics, three subenvironments (Fig. 1) are usually distinguished around the bay (Caline, 1982): (1) a sandy to silty estuarine system controlled by strong tidal currents; (2) an embayment characterized by sandflats and mudflats subjected to moderate tidal and wave energy; and (3) a sandy coast subjected to relatively high energy wave dynamics.
Studies in the 1970s on the Holocene infill of Mont-Saint-Michel Bay, from boreholes collected on reclaimed areas, concluded that successive stages of accelerating and decelerating transgression have occurred since 7 ka ago (Morzadec-Kerfourn, 1974). These events were manifested mainly by the development of peat bogs and vegetation changes. Recently, a very high resolution sequence stratigraphy analysis of the intertidal to subtidal wedge was performed: 50 Vibracores, 7 m long on average, were collected around Mont-Saint-Michel Bay, mostly in the middle to upper intertidal areas. Samples (juvenile shells, peat, organic-rich bulk sediment) were collected for accelerator mass spectrometry 14C dating at Poznaā Laboratory. Very high resolution seismic reflection data (600 km) were acquired (using an IKB-SEISTEC Boomer), mainly in the subtidal to lower intertidal domains. Some profiles reached the upper intertidal zones (Fig. 1).
The sequence stratigraphy analysis with this new data set included the entire Holocene infill of Mont-Saint-Michel Bay (Billeaud et al., 2007; Tessier et al., 2006), and demonstrated that the transgressive systems tract (TST) is very reduced in volume while the highstand systems tract (HST) makes up the main component of the infill. Below the present estuarine system, the HST consists of a tidal channel belt; because of a deep tidal ravinement base linked to the strong tidal currents, very little of the sedimentary record of the late Holocene is preserved. In contrast, the HST on the estuarine margins represents an almost continuous record of the late Holocene, i.e., of the past 6500 years. This study focuses on the very high resolution sequence stratigraphy of this HST preserved on the margins of the estuary.
VERY HIGH RESOLUTION SEQUENCE STRATIGRAPHY AND ARCHITECTURE
Sedimentary Successions
Vibracores collected on the sandflats in the embayment show successions of fining-upward sedimentary cycles, 1 m thick on average, each of which is defined at the base by an erosional surface. Each cycle is made up of coarse to very coarse shelly sands that pass upward to finer grained sand with mud drapes (Fig. 2A). The basal part of each cycle, according to sedimentary facies, including gutter casts, massive coarse sand, convex-upward shells, and upper flow regime planar bedding, is interpreted to be storm-dominated deposits. Hummocky cross-laminations due to winter storms, such as those in tidal flat deposits described by Yang et al. (2005), are not recognized, owing to the coarse grain size. The top part, characterized by low-energy heterolithic facies with frequent vertical burrows (Macoma, Nereis), is interpreted to be tide-dominated deposits. Four to five main cycles can be recognized.
Facies successions and interpretation from two Vibracores. A: Cherrueix 2 core, collected along embayment sandflat. B: Saint-Jean-le-Thomas 1 core, collected along sandy shoreline. In both sites, successions are made up of four to five metric sedimentary cycles, millennial in time scale (delineated with bold dotted lines). Oblique thin dotted lines indicate possible correlation between the two cores. Grain size: p—peat; c—clay; s—silt; fs, ms, cs—fine, medium, and coarse sand; g—gravels. cal yr B.P.–calibrated years before present.
Vibracores collected along the northeast sandy coast reveal successive cyclic variations of sedimentary facies, from peat layers to medium sands (Fig. 2B). The facies, including tidal bedding, well-sorted sand, mud clasts, and plant fragments, are interpreted to be back-barrier deposits (Billeaud et al., 2007). Peat and mud-rich facies are indicative of a well-sheltered back-barrier environment; sandy facies (massive fine-grained sands, thin eolian sand layers) correspond to higher energy marine conditions impinging on the back-barrier mudflat as a consequence of storm washover events. The coarsening-upward basal part of each cycle is interpreted to be the result of littoral barrier destabilization due to increased wave action. The fining-upward top part is associated with a phase of stabilization of the barrier (Fig. 2B). In all, four distinct cycles can be distinguished in the sedimentary successions.
Radiocarbon dating of the different peat layers from the successions of the northeast sandy coast (ca. 5923–6128, 4298–4330, 1566–1820, and 794–953 yr B.P.) indicate that the phases of barrier stabilization occurred with a periodicity of 1–2 ka. Throughout the sandflat successions, radiocarbon ages obtained on shells collected from the storm beds above the erosional surfaces (ca. 5582–5706, 4783–4918, 4087–4345, 2910–3163, and 1049–1232 yr B.P.) indicate a millennial recurrence of storm-dominated conditions.
Architectural Characteristics
Very high resolution seismic data reveal the architectural characteristics of the HST deposits. While in the estuary they are typically characterized by channelized geometries and lateral accretion packages; in the embayment and along the northeast coast, the HST geometry is mainly aggradational. In the entrance of Mont-Saint-Michel Bay, the HST is made up of subtidal banks.
Seismic profiles shot over the embayment sandflats show reflectors of high continuity alternating with intervals of much less continuous reflectors (Fig. 3A). This configuration is possibly related to the sedimentary cycles recognized in cores (Fig. 2A), indicating a lateral correlation between the cycles at the scale of the HST sandflat wedge. Seismic lines shot on the embayment mudflats reveal that the HST aggradational geometry is episodically interrupted by incisions several meters deep (Fig. 3B). According to the sedimentary facies (poorly bioturbated heterolithic tidal beddings over lying coarse-grained deposits with numerous mud balls) and their cross-shore orientation, these incisions should correspond to tidal inlets activated under the effect of hydraulic overloads. In the cores, thanatocoenoses of Hydrobia ulvae, a gastropod that lives in the lower salt marsh, overlie the incision surfaces. Hence the successive tidal incisions are related to storm periods that cause the destruction of the shelly barriers that shelter the salt marshes along the southwest coast of Mont-Saint-Michel Bay. The complete draining of the salt marshes after they have been inundated caused the hydraulic overloads. Four or five main incision phases can be identified on the seismic lines (Fig. 3B).
Seismic lines representative of different subenvironments of Mont-Saint-Michel Bay (except estuary). Susb—rocky substrate, TST—transgressive systems tract, HST—highstand systems tract, Gs—gas. Cores collected along profile are schematized and ages are indicated (calibrated 14C yr B.P.). Vertical scale in A is in two-way traveltime (twtt) (for P wave velocity of 1600 m/s in unconsolidated sediment) and meters. Thin gray dotted lines indicate highly continuous reflectors underlying probable lateral correlation of successive sedimentary cycles. In B and C, vertical scale is only indicated in meters.
In the northern subtidal entrances of Mont-Saint-Michel Bay, the tidal banks that form the seaward termination of the HST wedge are made of superimposed progradational sets, separated from each other by a flat erosional surface (Fig. 3C). As with the giant tidal dunes in Dover Strait (Le Bot and Trentesaux, 2004), these flat surfaces are interpreted to be the result of extreme storms. It is possible to correlate, at the scale of the bank area, five main progradational sets.
DISCUSSION
Our data indicate that the HST deposits preserved in Mont-Saint-Michel Bay record periodic environmental changes that can be correlated across the bay. These changes are attributed, in all cases, to storm impacts, which occur with a millennial time-scale periodicity. Four to five significant changes occurred. The times of these climatic crises were ca. 5500–5800, 4000–4500, ca. 3000, and 1000–1200 cal (calibrated) yr B.P., and match the time of Bond cold events (Fig. 4). These observations emphasized two main results.
Correlation of sedimentary cycles that are interpreted to be result of millennial-scale climate cycles, and that are preserved on margin of estuary in Holocene sedimentary infilling of Mont-Saint-Michel Bay. TST—transgressive systems tract.
These changes, by controlling primarily the hydrodynamics and the associated sediment supply, had a critical impact on coastal barrier behavior and tidal-flat construction. One of the best archives for climate events is represented by coastal barrier systems (Goy et al., 2003). Our study shows, for the first time, due to a very high resolution sequence stratigraphy in a complete sedimentary coastal basin, that, in spite of strong erosion processes occurring in a tidally dominated area, climatic events can be recorded in all settings that are directly or indirectly subjected to wave action. The Mont-Saint-Michel Bay is of interest because, despite a very high tidal range and strong tidal currents in the estuary, it is a fairly complete sedimentary basin with marginal and adjacent depositional environments. This composite organization appears to be the key for preservation of climate events in tide-dominated coastal areas.
The second main result of this study concerns the origins and atmospheric conditions that produced the sedimentary cycles. Increasing storm conditions in northwestern Europe are associated with positive North Atlantic Oscillations (NAO) (Hurrell and van Loon, 1997). However, Bond cold events (e.g., Bond et al., 1997) correspond to cool periods in the North Atlantic Ocean. It can be expected that during rapid climate changes of the late Holocene, atmospheric circulations favored positive NAO conditions in northwestern Europe. This hypothesis is confirmed by the studies carried out in Spain (Zazo et al., 2007), where climatic events are synchronous with those observed in Mont-Saint-Michel Bay. However, Spanish climatic events occur under arid conditions; those discussed here occur under more humid conditions. This anticorrelation corresponds typically to positive NAO conditions in northern Europe and to negative NAO conditions southward. This observation emphasizes the relationship between environmental changes recorded in coastal deposits and global climate fluctuations.
Some (e.g., Berger, 2008; Berger et al., 2002) have suggested that tidal components can modulate and/or force climate fluctuations, and long-term tidal cycles (1500–1800 years) are expected to be a possible cause of late Holocene rapid climate changes (Keeling and Whorf, 2000). The data described herein do not allow a consideration of this issue. However, it is possible that long-term tidal cycles modulate storm signatures in coastal deposits, because the impact of storm swells is enhanced during high spring tides. Hence, it could be expected that paroxysmal conditions corresponding to enhanced storminess during rapid climate-change events and maximum tidal dynamics during peaks of long-term cycles are responsible for the dramatic environmental changes that have occurred with a 1–2 ka recurrence in Mont-Saint-Michel Bay. Therefore, the environmental signal recorded at the scale of the Mont-Saint-Michel Bay basin may be the result of a combination of climatic and tidal components.
Acknowledgments
Billeaud was financially supported by the French Ministry of Research (Ph.D. grant). We acknowledge the financial support of Total E&P (Technology Centre in Pau, France) for complementary laboratory analyses, including 14C dating. We thank Graham Evans for English language corrections, as well as our colleagues at the M2C laboratory for assistance in the field. We also thank Bradley Opdyke and Bryan Hibbard for editorial advice, and an anonymous reviewer and Charles T. Blay for constructive reviews.
Footnotes
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- Received 10 April 2009.
- Revision received 19 June 2009.
- Accepted 30 June 2009.
- © 2009 Geological Society of America
