First occurrence of planktonic foraminiferal species Boliella adamsii as a marker for the Pleistocene-Holocene Boundary in the sea around Sumba Island

The expensive cost of Accelerator Mass Spectrometry radiocarbon dating (AMS 14C), the main tool for geochronology in Late Pleistocene-Holocene paleoclimate and paleoceanography study using marine sediment cores, hinders Indonesian researchers to produce competing research outputs. The first occurrences of planktonic foraminiferal species which have been applied as Pleistocene-Holocene Boundary markers in previous Quarternary marine sediment studies in the Indonesian region need to be confirmed as these species might actually occur since the Late Pleistocene. Quantitative foraminifera determination and oxygen isotope (δ18O) analysis of planktonic foraminiferal species Globigerinoides (Gs.) ruber have been carried out in marine sediment cores ST10 (off south Sumba) and ST14 (Sumba strait). δ18O of Gs. ruber was correlated to North Greenland Ice Core Project (NGRIP) Greenland Ice Core Chronology 2005 (GICC05) δ18O to determine the depth of the Pleistocene-Holocene Boundary (11653 BP) in core ST10 and ST14. Pleistocene-Holocene Boundary depth in core ST10 (31 cm) and ST14 (28 cm) are nearly coeval with the First occurrence (FO) of planktonic foraminifera Boliella adamsii, which at 34-35 cm depth interval in core ST10 and at 26-27 cm depth interval in core ST14. This indicated the practicality of Boliella adamsii FO as a Pleistocene-Holocene Boundary marker in the sea around Sumba Island, in case radiocarbon ages and proper planktonic foraminiferal δ18O data are not available. Future studies using a similar method in other Indonesian regions are needed to confirm the practicality of Boliella adamsii FO as a Pleistocene-Holocene Boundary marker in the Indonesian region.


Introduction
Geochronology, the study of time as it relates to Earth's history, is an important aspect of geological studies including paleoclimate and paleoceanography [1].Accelerator Mass Spectrometry radiocarbon dating (AMS 14 C) is highly utilized as the main tool to generate absolute age for geochronology in paleoclimate and paleoceanography studies using marine sediment cores, especially for Late Pleistocene-Holocene time scale.This method is able to produce a high resolution geochronology for geological studies covering the last 55,000 years using relatively small amounts of materials (less than 1 mg) [2].AMS 14 C instruments, however, are not available in most developing countries' research 1245 (2023) 012025 IOP Publishing doi:10.1088/1755-1315/1245/1/012025 2 facilities, incuding Indonesia.Researchers from these countries need to analyse their samples at overseas laboratories, usually in developed countries (i.e.United States, Germany, Japan).The price for AMS 14 C analysis in developed countries' laboratories is considered expensive by Indonesian standards.The analysis fee for each sample in those laboratories could cost around 500-750 USD or around 7,600,000-11,400,000 IDR (1 USD = 15,200 IDR) [3,4].The expensive price for AMS 14 C analysis makes it not affordable for Indonesian researchers, especially those with a limited budget such as university students and small university researchers.This limitation hinders them from producing competing research outputs.Due to the limited research budget, those researchers usually rely on planktonic foraminiferal biostratigraphy to know the geochronology of their research location even though it is less reliable compared to the AMS 14 C.
In Late Pleistocene-Holocene paleoclimate and paleoceanography studies, a geological event that can be easily recognized from biostratigraphy is the Pleistocene-Holocene Boundary.In the case when planktonic foraminiferal biostratigraphy is applied, the Pleistocene-Holocene Boundary can be indicated by the First Occurrence (FO) of Globorotalia fimbriata, Boliella adamsii, or Globigerinella calida [5,6].Previous Quarternary marine sediment studies in the Indonesian region applied either the FO of Globorotalia fimbriata [7,8], Boliella adamsii [9,10], or Globigerinella calida [11,12].However, there are arguments that those species might actually occur earlier, since the Late Pleistocene [5,13-15].These contradictive arguments need to be confirmed in order to increase the applicability of planktonic foraminiferal biostratigraphy in inferring the Pleistocene-Holocene Boundary.This research utilize quantitative planktonic foraminiferal distribution and foraminiferal oxygen isotope ( 18 O) records of marine sediment cores taken from off south Sumba and Sumba strait (Figure 1) to confirm the most practical biostratigraphic event as the Pleistocene-Holocene Boundary marker, especially in the sea around Sumba Island.Quarternary marine sediments in the sea around Sumba Island consist of calcareous pelagic and hemipelagic sediment [12,16].These sediments are rich in foraminiferal specimens [7,8,17,18] and contain transported pollen grains from nearby landmasses [19].Terrestrial materials are also present and their amount indicates the variability of Australian-Indonesian monsoon rainfall [19,20].

Data and Method
Two gravity sediment cores were collected from off south Sumba (ST10) and Sumba strait (ST14) as a part of the 2016 Widya Nusantara Expedition (E-WIN) using the Baruna Jaya VIII research vessel operated by the Research Center for Oceanography, Indonesian Institute of Sciences (LIPI) (now Research Center for Oceanography, National Research and Innovation Agency/BRIN) (Table 1).To produce planktonic foraminiferal distribution, foraminiferal identification in the 1 cm-thick sub-samples from the sediment cores is needed.Those sub-samples were taken at 2 cm intervals, resulting in a total of 191 sub-samples, 69 sub-samples from core ST10 and 122 sub-samples from core ST14.Sub-samples were weighed to ensure their weight consistency (~5 g each) and dried in a drying oven for ~6-8 hours at 40 o C afterward.Then, the sub-samples were washed on the top of a 230 mesh (63 m) sieve to clean the foraminiferal specimens from mud.The mud-free residue was then dried in a drying oven for ~6 hours at 60 o C. For quantitative foraminiferal identification, about 300 specimens of foraminifera were counted from each sub-sample as counting 300 specimens is considered enough to represent all species that might occur within [21].In the case of a sub-sample with very abundant foraminiferal specimens, each was split several times until each partition contains only around 300 foraminiferal specimens.Sub-samples were then observed under a stereo microscope in 50X magnification and foraminiferal specimens were determined based on previous highly-cited studies [5,15,22].All specimens in one partition were determined and counted while new species identified in other partitions were counted as one specimen.
The normalizations were done against the number of splits and the weight of samples (5 g).Samples preparation and determination were done at the Sedimentary Laboratory of the Research Center for Geological Disaster of the National Research and Innovation Agency (BRIN) and the Micropaleontology Laboratory of Institut Teknologi Bandung (ITB).
For planktonic foraminiferal  18 O analysis, ~6 specimens of Globigerinoides (Gs.) ruber sized 250-350 m (measured with a digital microscope) from 4-8 cm intervals were picked.Afterward, those specimens were washed with an ultrasonic cleaner for 15-60 seconds to clean the mud infillings and then dried in a drying oven for ~3-

Result and Discussion
Based on the correlation of Gs. ruber  18 O records of core ST10 to NGRIP GICC05  18 O records (Figure 2), the Pleistocene -Holocene Boundary was inferred at a depth of 31 cm.The FO of Boliella adamsii was detected at the depth interval of 34-35 cm which is relatively near to the depth of the Pleistocene-Holocene Boundary.Globigerinella calida was present throughout the entire core depth intervals.Globorotalia fimbriata first occurred at the depth interval of 90-91 cm, however, its occurrence was relatively inconsistent at the depth interval of 131 cm upward.
In core ST14, the correlation between Gs. ruber  18 O records and NGRIP GICC05  18 O records (Figure 3) indicate the depth of 28 cm as the Pleistocene-Holocene Boundary.The FO of Boliella adamsii was indicated relatively near to the depth of the Pleistocene-Holocene Boundary, at the depth interval of   In both core ST10 and ST14, the FO of Boliella adamsii was detected at the adjacent depth or nearly coeval to the Pleistocene-Holocene Boundary.This depth adjacency indicates that the FO of Boliella adamsii is applicable as the marker of the Pleistocene-Holocene boundary at least in the sea around Sumba Island.On the other hand, the use of Globorotalia fimbriata and Globigerinella calida FO as a Pleistocene-Holocene boundary markers is not recommended.These species were most likely occurred at least since the Late Pleistocene which was also suggested by previous studies [5,13,15].
Similar studies which combine the planktonic foraminiferal biostratigraphic event, particularly Boliella adamsii FO with oxygen isotope ( 18 O) chronology in other Indonesian regions are needed in the future.These future studies will confirm the applicability of Boliella adamsii FO as a Pleistocene-Holocene Boundary marker in the Indonesian region.For now, new evidence of Boliella adamsii FO applicability as a Pleistocene-Holocene Boundary marker from this study hopefully will assist Indonesian researchers, especially the low-budgeted students and small university researchers to study the Late Pleistocene-Holocene paleoclimate and paleoceanography using marine sediment cores.

Conclusion
Based on its depth adjacency (near coeval) to the depth of Pleistocene-Holocene Boundary in both core ST10 and ST14, the FO of Boliella adamsii is applicable as the marker for the Pleistocene-Holocene Boundary in the sea around Sumba Island if radiocarbon ages and proper planktonic foraminiferal  18 O data are not available.The use of FO of other previously applied species (i.e.Globorotalia fimbriata and Globigerinella calida) as a Pleistocene-Holocene Boundary marker is not recommended as those species were most likely occurred at least since the Late Pleistocene.Future studies with a similar method in other Indonesian regions are also needed to confirm the applicability of Boliella adamsii FO as a Pleistocene-Holocene Boundary marker in the Indonesian region.

Figure 1 .
Figure 1.Location of two marine sediment cores used in this study: (A) ST10 and (B) ST14.
1245 (2023) 012025 IOP Publishing doi:10.1088/1755-1315/1245/1/012025426-27 cm.Globigerinella calida occurred in the entire core depth intervals while Globorotalia fimbriata first occurred at the depth interval of 96-97 cm.Similar to core ST10, the occurrence of Globorotalia fimbriata was also relatively inconsistent at the depth interval of 97 cm upward.

Figure 2 .
Figure 2. Comparison of biostratigraphic events in core ST10 to the Pleistocene-Holocene Boundary inferred from the correlation between ST10 Gs. ruber  18 O records and NGRIP GICC05  18 O records.

Figure 3 .
Figure 3.Comparison of biostratigraphic events in core ST14 to the Pleistocene-Holocene Boundary inferred from the correlation between ST14 Gs. ruber  18 O records and NGRIP GICC05  18 O records.

Table 1 .
Information of sediment cores used in this study.
Appendix B Detailed distribution of the Pleistocene-Holocene Boundary marker species: Boliella adamsii, Globorotalia fimbriata, and Globigerinella calida in core ST10.