Intra-annual Variability in Benthic Foraminiferal Abundance in Sediments of Disenchantment Bay, an Alaskan Glacial Fjord

Figures & data

Figure 1 Location map of Disenchantment Bay in the northern Gulf of Alaska. Cores AH04-YB PC18, and MC 2-3 (Fig. 2) and 2-6, as well as EW0408 76MC, 72TC, and 72JC, were collected at Station 1. Core AH04-YB MC 1-2 was collected at Station 2.

Figure 2 Nested core chronology for a suite of cores collected at Station 1. Annual deposition is marked by a winter diamicton and a summer laminated mud. Episodic sand beds (turbidites) appearing in multiple cores were used as datums for stratigraphic correlation. Visual core descriptions, as well as x-radiograph images (inset) were used to distinguish bounds of diamicton and sand beds. SWI indicates the sediment-water interface at time of collection.

Figure 3 SEM Photomicrograph of foraminifera species Elphidium excavatum f. clavatum (A), Elphidium subarcticum (B), Elphidium frigidum (C), and Textularia earlandi (D, E).

Figure 4 Multicores showing a 3-point moving average of stained and total abundance for Elphidium spp. and T. earlandi. These moving averages are based on raw counts normalized to 1 g of fine-sediment. The percentage of stained individuals relative to total specimens of that taxa is also presented. These percentages are based on the raw abundance counts. Lithology and seasonal interpretation for both multicores are included.

Table 1 (a, b) Raw species counts from Multicores MC1-2 and MC2-6. Species references are given in Appendix. For the purposes of this study, all counts were normalized to 1 g fine-sediment. Mass of sample examined indicates the amount of material analyzed after splitting of the sample. Abundances were normalized to this mass. Parentheses indicate specimens stained at the time of collection.

Table 2 Raw species counts from jumbo-core 72JC. All counts used in Figure 5 are normalized to 1 g fine-sediment. Species references are given in Appendix.

Figure 5 Jumbo core (EW0408-72JC) with total abundance of Elphidium spp. and T. earlandi normalized to 1 g fine-sediment, bulk density, core lithology, and chronology.

Figure 6 Sediment mass flux, bulk sediment carbon to nitrogen concentrations, and ²³⁴Th activity at Station 1 (76MC) during one full year. Error bars in radioisotope activity represent the total uncertainty in gamma spectroscopic measurements, and the error in mass fluxes represents propagation of the 5–10% uncertainty in activities into time units (days since core collection).

Figure 7 Schematic model of foraminiferal migration and development relative to sedimentation rate during the four month long meltwater season in Disenchantment Bay. (A) Sedimentation rates and foraminiferal abundance are based on ²³⁴Th chronology and counts from multicores. Abundances observed in the core parallel the increasing population of reproducing adults throughout the meltwater season. (B) This conceptual diagram depicts the migration near the sediment surface of both juvenile and reproducing foraminifera as sediment accumulates to produce the observed intra-annual variability. Within winter seasons, populations consist predominantly of juvenile individuals and very few preserved tests. Later in the meltwater season (September), reproducing adults replace the juvenile population, undergo a reproduction event, and produce a subsequent juvenile population.