1. The Origins of Bermuda

Emergence of the islands

The story of Bermuda began tens of millions of years ago with the eruption of a cluster of volcanic seamounts, including the main Bermuda Seamount (Figure 1a),  in the North Atlantic ocean.  Following a long period of  erosion the seamounts were truncated below present sea level. They were then blanketed in accumulations of carbonate sediments composed of the skeletal remains of marine life which had colonised the submerged edifice (Figures 1b and 1c). Only on the largest seamount – the main Bermuda seamount  – did shallow waters persist long enough for  permanent shoals, cays and dunes to form, which eventually became cemented into the Bermuda Islands (Figures 1d and 1 e).

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Figure 1a. Eruption of the Seamount. Eruption of magma from beneath the ocean floor formed the Bermuda  volcanic seamount which emerged from the sea as a volcanic island more than 30 million years ago.
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Figure 1b.  Erosion of the seamount.  Once eruptions had ceased, the Bermuda volcanic island was gradually eroded away by ocean waves. The product of this erosion would have been beaches and seabed sediments composed of black volcanic sand.
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Figure 1c. Generation of carbonate sediment.  By 20 million years before present, it is has been calculated (VO1) that all remnants of the volcanic island had been removed by erosion. Encircled by reefs, the Bermuda platform became a sheltered mid-ocean refuge for shallow-water marine life including corals, mollusks, forams and algae. Over time, the accumulation of their broken skeletons was responsible for burial of the whole platform with carbonate sand.
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Figure 1d. The creation of  islands by dunes. Currents and waves concentrated the platform sands into shoals and then low lying islands. Dunes began to form where wind-blown sand, transported from the beaches, was trapped among vegetation above the high tide level. Small dunes developed into large landward-advancing mobile dunes during periods  of high sand supply. The islands continued to grow despite episodic storm damage which was counteracted by the ongoing generation of skeletal sand in the “carbonate factory”.
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Figure 1e. Multiple cemented dune ridges form permanent islands.  Episodes of dune activity were limited to periods  when sea levels were  relatively high and the platform was flooded. These alternated with long periods of dune inactivity and cementation, when sea levels were low and the platform was exposed as a large forested plateau.  This cycle of events was repeated many times throughout the Pleistocene Epoch, resulting in the cemented dune ridges which form the Bermuda Islands of today.

Pleistocene sea level oscillations

Climatic cycles of the Pleistocene Epoch, and associated advances and retreats of the great continental ice sheets, dramatically affected global sea levels through a range of over 100 m (320 ft) (Figure 8d, Chapter 8). These oscillations in sea level  played a pivotal role in the generation and distribution of carbonate sediments on the Bermuda platform.

During episodes when sea level flooded the Bermuda platform, carbonate sand was being generated from the broken skeletal remains of marine organisms, and was accumulating on beaches and intermittently in dunes. When sea levels fell by more than approximately 20 m (65 ft) from their maximum level, these processes ceased as the platform emerged in the form of a large island – Greater Bermuda (Figure 1f) . These periods of emergence  are recorded in the geological record by extensive well-developed fossil soils, or terra rossa palaeosols, which are interlaced through the limestone deposits (Figure 1g).

There were many climatic cycles during the Pleistocene Epoch, which lasted approximately 2.5 million years.  These cycles became larger in amplitude over the last 1 million years or so, which is the time span within which the limestone islands of the Bermuda  are considered to have formed (HE6).  At a minimum there is evidence of seven of these cycles on Bermuda. They are represented by the five limestone formations (and their sub-divisions), with intervening palaeosols, which have been identified and mapped  throughout the islands (Chapter 5).

Low sea level forested platform
Figure 1f. Emergence of the seamount at low sea levels. Greater Bermuda emerged as a forested island during glacial periods of the Pleistocene Epoch when sea levels were lowered by up to 100 m (320 ft) below present sea level. The land area was more than 10 times larger than that of today’s islands. Each of these periods was recorded by the widespread development of a mature terra rossa soil.
High sea level new dunes
Figure 1g. Flooding of the seamount at high sea levels. Sea level rise during Interglacial periods of the Pleistocene Epoch resulted in flooding of the Bermuda Platform and its subsequent re-colonisation by marine plants and organisms including corals. Their skeletal remains were broken down into carbonate sand which blanketed the platform. Towards the end of a given interglacial period, a fall in sea level or minor oscillations, exposed this sand on wide beaches from which it was swept by the wind into the dunes which formed Bermuda.