Supplemental Figures

Figure S1 – Stratigraphy from core transect T1 (left) on Thompson Island and core PdI-1 (right; see

Figure 2 in manuscript for core/transect locations). Note Boston Blue Clay was found extensively

in transect T1 (shown in picture lower right).

PdI-2 and PdI-3 terminated in shallow (<100 cm deep) gravel deposits, thus were not used in the

lithologic or basal peat analyses presented here.

Figure S1 (continued) – Stratigraphy from core transects T2 (left) and T3 (right) on Thompson

Island (see Figure 2 in manuscript for core locations. Boston Blue Clay was found extensively in the

transect T1, and therefore is postulated to exist at depth in these transects, however further studies

are needed for verification.

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Figure S2 – Top: Core lithology, organic content, and bulk density from Thompson Island core TI-3,

displaying abrupt shift from low saltmarsh Spartina alterniflora at the top to high saltmarsh S. patens at

depth. Organic content shows organic rich mud that contains Spartina alterniflora roots is ~20% at the

surface (bulk density ~0.2-0.4 g/cm3), increasing with depth where the macrofloral roots indicate

presence of Spartina patens or consolidated peat (organic content >40%; bulk density <0.2 g/cm3).

Bottom: select core data from other Thompson Island and Peddocks Island (PdI) cores show similar

variations in organic content. Note the depth for PdI-1 is different.

Figure S3 - Conceptual model for development of saltmarshes on Thompson Island using

previously published data and radiocarbon results from this study. These isolated marshes (dated

to ≥ 4.2 ka) have filled in topographic lows, postulated to be deltaic and glacial drumlin deposits

from the advance and retreat of the Laurentide Ice sheet4. Saltmarshes on Peddocks Island are

thought to have developed in a similar fashion, though isolated by spits, dated at ~2.8 ka.

References

1Hein CJ, FitzGerald DM, Buynevich IV, et al. (2014) Evolution of paraglacial coasts in response to changes in

fluvial sediment supply, Martini IP, Wanless HR (eds), Sedimentary Coastal Zones from High to Low Latitudes:

Similarities and Differences, Geological Society, London, Special Publications 388, 247–280.

2Kenney TC (1964) Sea-level movements and the geologic histories of the post-glacial marine soils at Boston,

Nicolet, Ottawa, and Oslo. Géotechnique 14: 203–230.

3This study

4Kaye CA (1982) Bedrock, and quaternary geology of the Boston area, Massachusetts. U.S.A. Geol Surv Am Rev

Eng Geol 5: 25–40.