Marc P. Hijma

Timing and magnitude of the sea-level jump preluding the 8200 yr event

Evidence from terrestrial, glacial, and global climate model reconstructions suggests that a sea-level jump caused by meltwater release was associated with the triggering of the 8.2 ka cooling event. However, there has been no direct measurement of this jump using precise sea-level data. In addition, the chronology of the meltwater pulse is based on marine data with limited dating accuracy. The most plausible mechanism for triggering the cooling event is the sudden, possibly multistaged drainage of the Laurentide proglacial Lakes Agassiz and Ojibway through the Hudson Strait into the North Atlantic ca. 8470 ± 300 yr ago. Here we show with detailed sea-level data from Rotterdam, Netherlands, that the sea-level rise commenced 8450 ± 44 yr ago. Our timing considerably narrows the existing age of this drainage event and provides support for the hypothesis of a double-staged lake drainage. The jump in sea level reached a local magnitude of 2.11 ± 0.89 m within 200 yr, in addition to the ongoing background relative sea-level rise (1.95 ± 0.74 m). This magnitude, observed at considerable distance from the release site, points to a global-averaged eustatic sea-level jump that is double the size of previous estimates (3.0 ± 1.2 m versus 0.4–1.4 m). The discrepancy suggests either a coeval Antarctic contribution or, more likely, a previous underestimate of the total American lake drainage.

From river valley to estuary: the evolution of the Rhine mouth in the early to middle Holocene (western Netherlands,Rhine-Meuse delta)

The aim of this paper is to reconstruct the evolution of the early to middle Holocene Rhine-Meuse river mouths in the western Netherlands and to understand the observed spatial and temporal changes in facies. This is achieved by constructing three delta wide cross-sections using a newly accumulated database with thousands of core descriptions and cone penetration test results, together with a large set of pollen/diatom analyses and OSL/14C-dates. Most of the studied deposits accumulated in the fluvial-to-marine transition zone, a highly complex area due to the interaction of terrestrial and marine processes. Understanding how the facies change within this zone, is necessary to make correct palaeogeographic interpretations. We find a well preserved early to middle Holocene coastal prism resting on lowstand valley floors. Aggradation started after 9 ka cal BP as a result of rapid sea-level rise. Around 8 ka most parts of the study area were permanently flooded and under tidal influence. After 8 ka a bay-head delta was formed near Delft, meaning that little sand could reach the North Sea. Several subsequent avulsions resulted in a shift from the constantly retreating Rhine river mouth to the north. When after 6.5 ka the most northerly river course was formed (Oude Rijn), the central part of the palaeovalley was quickly transgressed and transformed into a large tidal basin. Shortly before 6 ka retrogradation of the coastline halted and tidal inlets began to close, marking the end of the early-middle Holocene transgression. This paper describes the transition from a fluvial valley to an estuary in unprecedented detail and enables more precise palaeo-reconstructions, evaluation of relative importance of fluvial and coastal processes in rapid transgressed river mouths, and more accurate sediment-budget calculations. The described and well illustrated (changes in) facies are coupled to lithogenetic units. This will aid detailed palaeogeographic interpretations from sedimentary successions, not only in the Netherlands, but also in other estuarine and deltaic regions.

The contribution of glacial isostatic adjustment to projections of sea-level change along the Atlantic and Gulf coasts of North America

We determine the contribution of glacial isostatic adjustment (GIA) to future relative sea-level change for the North American coastline between Newfoundland and Texas. We infer GIA model parameters using recently compiled and quality-assessed databases of past sea-level changes, including new databases for the United States Gulf Coast and Atlantic Canada. At 13 cities along this coastline, we estimate the GIA contribution to range from a few centimeters (e.g., 3 [−1 to 9] cm Miami) to a few decimeters (e.g., 18 [12–22] cm, Halifax) for the period 2085–2100 relative to 2006–2015 (1−σ ranges given). We provide estimates of uncertainty in the GIA component using two different methods; the more conservative approach produces total ranges (1−σ confidence) that vary from 3 to 16 cm for the cities considered. Contributions from ocean steric and dynamic changes as well as those from changes in land ice are also estimated to provide context for the GIA projections. When summing the contributions from all three processes at the 13 cities considered along this coastline, using median or best-estimate values, the GIA signal comprises 5–38% of the total depending on the adopted climate forcing and location. The contributions from ocean dynamic/steric changes and ice mass loss are similar in amplitude but with spatial variation that approximately cancels, resulting in GIA dominating the net spatial variability north of 35°N.

The Transgressive Early–Middle Holocene Boundary: The Case for a GSSP at Rotterdam, Rhine Delta, North Sea Basin

Postglacial sea-level rise at the start of the Holocene continued to drown continental shelves around the world. By the early–middle Holocene transition, deltas and other coastal systems had begun to stabilize their positions, which have since been maintained. The last major acceleration of sea-level rise occurred between 8.5 and 8.2 ka, due to the largest meltwater pulse from a single source area, released from the thawing Laurentian ice sheet in the Hudson Bay area. This event left a marked transgressive impact on river-mouth sedimentary sequences around the globe, exemplified in the Rhine Delta (North Sea, The Netherlands) from boreholes and underground exposures in the city of Rotterdam and its megaharbour. What ended as the 8.2 ka climatic event actually began as a freshwater release at ~8.45 ka: it should therefore be properly regarded as an 8.5–8.2 ka event. In contrast to the 8.2 ka climatic event, which was temporary, globally highly variable, and commonly insignificantly registered, and which only indirectly affected sedimentation, the sea-level imprint of the freshwater release was permanent, circumoceanic, and predictably spatially variable, and had direct impacts on sedimentation on both sides of the migrating coastline. Consequently, the water release left lithostratigraphic- and environmental-event boundaries in coastal sequences around the world, in the zone where Holocene accumulations are thickest and functional subdivision is architecturally most important. For these reasons, the sea-level signal of the 8.5–8.2 ka event should be considered as the beginning of a formalized Middle Holocene, and not the somewhat-later 8.2 ka cold spell maximum over Greenland, as is currently being proposed elsewhere. In that context, the transgressive contact found at the base of the Rhine Delta at Rotterdam is presented as a potential GSSP (8450 ± 44 cal BP).

Comment on: Mid-Holocene water-level changes in the lower Rhine-Meuse delta (western Netherlands): implications for the reconstruction of relative mean sea-level rise, palaeoriver-gradients and coastal evolution by Van de Plassche et al. (2010)

In May 2009, Orson van de Plassche sadly passed away. In a paper that was partially written after his death, new data and a revision of an existing sea level curve are published for the Rotterdam area (Van de Plassche et al., 2010). Our comment concerns two topics they address in their discussion section: 1) connection of their revised Rotterdam relative sea-level curve for the period 7900-5300 cal yr BP (Jelgersma, 1961; Van de Plassche, 1982; 1995; Berendsen et al., 2007; Van de Plassche et al., 2010) to the sea-level curve for the same area for the period 9000-7500 cal yr BP (Hijma and Cohen, 2010); 2) The role of the river gradient on our calculation of a sea-level jump that occurred between 8450-8250 cal yr BP (Hijma and Cohen, 2010).