THE SEDIMENT RECORD AS AN INDICATOR OF CHANGING OCEAN-LAGOON DYNAMICS, BOKA ASCENSION, CURACAO

Characterizing environmental changes over time provides insight into how conditions may change in the future. Two sediment cores (BA4, BA5) were near the lagoon edge of Boka Ascension (Curaçao), an inlet incised through an uplifted Pleistocene marine terrace. Ten samples from BA5 and eight samples from BA4 were analyzed for sedimentological and foraminiferal characteristics. A cluster analysis identified three biofacies, which were combined with sedimentological data to create three depositional facies (DF): Higher energy normal marine conditions (DF I), boka energy regime change event (DF II), and oscillating energy conditions (DF III).

BA5 shows a change from normal high energy conditions (DF I) to oscillating energy conditions (DF III). This transitions to a pulse of poorly-sorted sand (DF II) indicating a sudden change in energy regime of the boka. After this, the boka returns to normal conditions (DF I). These normal conditions correlate with the oldest layers of BA4, which then transitions into a second oscillating sequence (DF III) before returning to normal conditions (DF I). These sedimentological changes reflect a change in the exchange of water between the ocean and the boka. This change is further reflected in the foraminiferal record; foraminiferal diversity is substantially higher before DF II (avg. Fisher’s alpha = 4.98) than after (avg. Fisher’s alpha = 1.20). Depositional Facies II likely represents a boka breakthrough event in which the previously restricted lagoon (pre-DF II) rapidly opened resulting in a greater, sustained ocean influence. This change likely created inhospitable conditions for many lagoon species, resulting in the observed lower diversity.

This hypothesis of change in boka morphology interpreted from the sediment record is further supported through the examination of satellite imagery. These images demonstrate that the boka’s morphology has changed over at least the past fifteen years. The changes in energy inferred from the sediment record are evidence of a changing boka morphology and its degree of exchange with the ocean.