Paleoseismology of a Currently Creeping Section of the Eastern Aleutian Subduction Zone: Collaborative Research with University of Rhode Island and U.S

Paleoseismology of a Currently Creeping Section of the Eastern Aleutian Subduction Zone: Collaborative Research with University of Rhode Island and U.S. Geological Survey

External Grand Award Numbers: G16AP00149

Investigators: Simon Engelhart1, Christopher Vane2

1. Department of Geosciences, University of Rhode Island, 336 Woodward Hall, 9 East Alumni Avenue, Kingston, RI 02881, USA, [email protected] Tel: +1 401 874 2187 Fax: 401 874 2190

2. British Geological Survey, Environmental Science Center, Keyworth, Nottingham, NG12 5GG, UK, [email protected] Tel: +44 (0) 115 936 3017

Start date: July 1st 2016 End date: September 30th 2017

Research supported by the U.S. Geological Survey (USGS), Department of the Interior, under USGS award numbers G16AP00149. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government. Abstract

We described stratigraphy and collected samples at four sites on Unga Island: Ocean Beach; Tombolo Marsh; Bull Spit; and Unga Bog. Field methods involved the use of gouge cores, outcrops, and pits to undertake reconnaissance of stratigraphy. We collected core samples using Russian and Fat Gouge hand driven coring devices at core locations that were representative of the overall stratigraphy at each study site. Analysis of the cores resulted in us obtaining 20 new AMS radiocarbon dates that constrain the timing of sediment deposition at all four sites.

Our stratigraphic data indicates no evidence for marine inundation from tsunami or storms, or land-level changes greater than 0.3m. Tombolo Marsh has been in existence for ~6000 years, although the relationship of the site to sea level during this entire record remains unclear. However, the stratigraphy and subsequent paleoenvironmental interpretations at Ocean Beach and Bull Spit indicates that these sites formed and existed in close proximity to sea level during the last ~750- 1000 years and would be expected to record marine inundation or land-level changes greater than 0.3m. Based on this evidence and the similarity of elevations between the sites, we suggest that Tombolo Marsh should also have been a suitable recorder for land-level changes and marine inundation during at least the same time period. Despite the availability of suitable sedimentary environments for recording marine inundation over the past ~750-1000 years, we find no evidence for the 1788 earthquake and tsunami reported by Russian settlers as having inundated Unga Island to heights of up to 50m. Unga Island is now the second site in the Shumagin Islands to record no geological evidence of this earthquake and tsunami, as well an absence of evidence from Sanak Island to the west. This raises further questions over the lack of preservation of this tsunami at multiple sites or, more likely, raises concerns over the validity of the second-hand reports of this earthquake and tsunami in the Shumagin Islands.

Context

Frequent great (Mw8-9.2) earthquake rupture and associated destructive tsunami (e.g., 1946, 1957, 1964, 1965) have occurred ruing the 20th century on the Alaska- Aleutian subduction zone (Sykes, 1980, Carver and Plafker, 2008). GPS can be used to understand modern rates of coupling, but it is unclear if these are relevant over periods longer than decades. Indeed, the short time period of these records precludes the development of unique interpretations of the data (Wang et al., 2012). While some evidence suggests that modern geodetic measurements may be consistent with prior rupture behavior (e.g., Moreno et al., 2010; Loveless and Meade, 2011; Witter et al., 2014), there is also evidence that rupture boundaries are non-persistent over geological time (e.g., Briggs et al., 2014; Ely et al., 2014). With continued debates over these questions from paleoseismic records, geodetic measurements, and fault modeling (e.g., Fournier and Freymueller, 2007; Noda and Lapusta, 2013; Witter et al., 2014), targeted paleoseismic studies at sites of interest remains the sole means to accurately evaluate long-term hazard (e.g., Thatcher 1989, Shennan et al., 2014)

Despite ongoing collaborative effects commencing in 2010 (e.g., Briggs et al., 2014; Witter et al., 2014; Kelsey et al., 2015; Nelson et al., 2015; Witter et al. 2016, 2018), detailed paleoseismology records of the Alaska-Aleutian megathrust are still limited, particularly west of the 1964 rupture. Our study site in the northern Shumagin Islands is located within the sparsely studied area of the Aleutians west of Kodiak Island. The Shumagin Islands sit as a possible segment boundary and modern GPS suggests that the region is creeping. Witter et al (2014) demonstrated no evidence for large (>0.3m) vertical land-level changes or significant tsunami deposits in the last ~3800 years at Simeonof Island, suggesting that earthquakes similar to the 20th century record (M7-7.5) may be sufficient to release the stored elastic strain. However, this evidence is contradictory to Russian accounts of a large tsunami (>30m) in 1788 that is reported to have inundated Unga Island (Soloviev, 1990; Lander, 1996). Evidence for this earthquake has been identified within the Kodiak segment (e.g., Briggs et al., 2014; Shennan et al., 2014, 2018; Nelson et al., 2015) but is absent from Simeonof Island (Witter et al., 2014) and Sanak Island (Engelhart et al., 2016). Witter et al. (2014) identified that further fieldwork was needed in this area to evaluate the evidence for a great or giant Shumagin scenario that would result in a change to current Mmax (M8) and associated recurrence intervals in seismic hazard assessment (e.g., Wesson et al., 2007).

We therefore set out to evaluate the paleoseismic history of Unga Island to address research questions including:

1. Are modern geodetic measurements consistent with the paleoseismology?

2. If evidence for earthquakes and/or tsunamis are found, what are the recurrence intervals for these events near Unga Island?

Project deliverables included:

1. Collection of sediment cores from sites on Unga Island to analyze evidence for land-level changes and/or tsunami deposits.

2. Analysis of geochemistry samples.

3. Analysis of AMS C-14 dates to produce a chronology for Holocene environmental changes on Unga Island.

4. Presentation of results at scientific meetings.

5. Submission of results to peer-reviewed journals

1. Summary of deliverables We described stratigraphy and sampled four sites on Unga Island: Ocean Beach, Tombolo Marsh, Bull Spit, and Unga; Figure 1). Field methods involved the use of gouge cores to undertake reconnaissance of stratigraphy. We collected core samples using Russian and Fat Gouge hand driven coring devices at core sites that were representative of the overall stratigraphy at each site. Analysis of the cores resulted in us obtaining 20 new AMS radiocarbon dates that constrain a chronology for Holocene environmental change on Unga. The AMS radiocarbon dating confirms our field correlations based on marker tephra horizons.

We assessed whether a new organic geochemistry technique could identify the presence of sterols associated with the introduction of cattle to the island that could be used as a chronological marker for ages younger than 1650 CE. We have presented our results as part of summary presentations on our Shumagin Islands work at the European Geophysical Union in April 2018 (Engelhart et al., 2018) and will submit a review paper on the history of the 1788 earthquake that incorporates this data for peer review by September 2019. All data generated in this project will be included in, or attached as supplementary information, to this planned publication.

2. Challenges and Research Plan Modifications Our initial field plan was to target Archeredin Bay to the west of the sites described here. We had access to a helicopter through collaboration with the United States Coastguard to transport us to the site, which is unable to be reached reliably by boat. However, due to heavy weather for the first three days of the research mission, it was not possible to land on Unga despite multiple attempts. Due to time constraints, the United States Coastguard crew had to leave to return to duty on Kodiak Island. We, therefore, changed our plan and chartered a fishing vessel to transport us to Delarof Harbor, enabling access to sites described in the initial Russian reports of inundation during 1788 and trench facing sites at Ocean Beach.

3. Methods

3.1 Core Descriptions Grain size, sedimentary structures, contacts, thickness, and lateral and vertical facies changes were described in the field using general stratigraphic methods in combination with the Troels-Smith (1955) method for describing organic-rich sediment.

3.2 Surveying to sea-level datum Elevational accuracy is crucial for all our mapping and sampling. To establish vertical orthometric and tidal data at each site, we pursued two strategies. Firstly, we leveled cores, pits, and topographic elevations with an RTK GPS instrument. Data collected by the GPS base station was post-processed to obtain North American Vertical Datum 88 (NAVD88) orthometric elevations. To establish elevations with respect to a tidal datum, we installed water pressure sensors at Delarof Bay to capture the shape and timing of the tidal curve for comparison to the nearest tide gauge at Sand Point. We needed to use both orthometric and tidal-datum strategies because the link between NAVD88 orthometric and NOAA tidal data is not well established in Alaska (e.g., Kemp et al., 2013a).

3.3 Chronology We picked plant macrofossil samples from selected cores for AMS 14C dating (Table 1). We focused on samples that can tightly constrain the age of sediment deposition, such as tidal herb stems entombed in mud or sand and seeds from terrestrial plants (e.g., Kemp et al., 2013b). Multiple 14C ages above and below key contacts related to land-level changes and tsunami deposits allow stratigraphic correlation among sites.

3.4 Geochemistry To determine up to eleven individual sterol biological marker compounds in Unga sediments / soils. These compounds include cholestane (5α-cholestane), coprostanol (5β-cholestan-3β-ol), 5β-epicoprostanol (5β-cholestan-3α-ol), cholesterol (cholest- 5-en-3β-ol), 5α-cholestanol (5α-cholestan-3β-ol), coprostan-3-one (5β-cholestan-3- one), campesterol (24α-methyl-5-cholesten-3β-ol), stigmasterol (3β-hydroxy-24- ethyl-5,22-cholestadiene), fucosterol ((3β,24E)-stigmasta-5,24(28)-dien-3-ol), β- sitosterol (24-ethylcholest-5-en-3β-ol) and 5β-stigmastanol (24α-ethyl-5α- cholestan-3β-ol); chemical structures are presented in Figure 2.

Separate surrogate standards of deuterated cholesterol (cholesterol-2,2,3,4,4,6-d6) and 5α-cholestan-3b-ol-d5 were made containing 50 ng/μL of each compound dissolved toluene:pyridine (20:1 v/v). Internal standards of androstanol (126ng/µL) and deuterated cholestane-d6 (117ng/µL) were made separately dissolved toluene:pyridine (20:1 v/v). Sediment samples were freeze-dried, not sieved and ground to a fine powder in a stainless-steel ball mill (Retsch PM400). 10g of sample was placed on a watch-glass and spiked with 50µL of the surrogate standard and allowed to equilibrate at ambient temperature for 18-24 hours. The samples were subsequently mixed with a dispersant, anhydrous sodium sulphate that had been preheated at 550°C for 12 hours and allowed to cool. Thereafter, sediments were extracted with methanol/dichloromethane (MeOH/DCM) (1:1 v/v) using an accelerated solvent extraction system ASE 200 (Dionex) operated at a temperature of 100°C and a pressure of 1500psi. Activated copper powder (2 g) was added to remove elemental sulphur. The extract was reduced in volume to approximately 3mL using a Turbovap at 30°C, then to dryness using a gentle stream of dry nitrogen and immediately reconstituted in 1mL of acetone.

The samples were cleaned up using silica gel cartridge (Bond Elut, HF Mega BE – SI, 10gm 60mL, Agilent Technologies). Cartridges were preconditioned with three column volumes of hexane:DCM (3:1 v/v) and the extract transferred onto the surface of the column using a glass pipette. It was first eluted with 40mL hexane/DCM (3:1 v/v), then 80 ml DCM and finally with 60 ml acetone/DCM (3:7 v/v). The latter two fractions were combined, the solvent evaporated using a Turbovap at 30°C and the residue dissolved in 1mL acetone prior to quantitative transfer to a glass vial (1.75mL). Acetone was removed by evaporation with a gentle stream of dry nitrogen, immediately reconstituted in 0.9mL pyridine and 50µL of each internal standard added. An 80µL aliquot was transferred to a 250µL glass vial insert containing 80µL of pyridine and silylated by heating in an oven at 60 °C for 60min with 40μL of N, O- bis-(trimethylsilyl)trifluoroacetamide (BSTFA) with 1%TMCS (Sigma Chemical Co.) and allowed to stand at ambient temperature for approximately 18 hours prior to GCMS analysis

Sterols were analyzed using a Varian CP3800 series gas chromatograph (GC) directly coupled with a Varian 1200L triple Quadrupole MS/MS system (GC/MS). Sample injection (1.0 μl) was in splitless mode. Compounds were separated using an Agilent DB-5MS column (30m length × 0.25mm i.d. × 0.1μL film thickness). The oven temperature was programmed from 60°C (1min isothermal) to 250°C at 20 °Cmin−1 then to 320°C at 4°C min−1 and held isothermally at 320°C for 20 min. The mass spectrometer was operated at 70 eV in single ion monitoring mode with helium as carrier gas at a flow rate of 1 mL/min. Data acquisition was carried out using a Varian MS workstation v6.5. Peak assignments were made by comparison with published mass spectra and mass spectra and retention times of authentic standard compounds (Figure 1).

4. Results

4.1 Chronological data Our new AMS radiocarbon ages are shown in Table 1. As described below, the absence of stratigraphic evidence for either land-level changes or marine inundation led us to focus ages on identifying the age of sequences and on specific tephra that could be used for stratigraphic correlations.

4.2 Stratigraphy

4.2.1 Ocean Beach The site at Ocean Beach is composed of a beach ridge plain varying in width from approximately 30 to 225 m and fronted by an erosional scarp on the largest berm. No similar sized berms to the berm at the front of the ridge are found further inland unlike at other sites along the Alaska-Aleutian megathrust (e.g., Witter et al., 2018). The beach is formed of boulders in the upper shoreface with sand-sized sediment in the lower shoreface. Sediment thickness above rounded cobbles that underlie the beach ridge plain varies from approximately 0.1 m to 1.0 m and generally thins landward from the modern beach. Radiocarbon ages on plant macrofossils overlying the lower cobble unit indicate that this most prominent ridge formed between 700 and 1000 calibrated years before present (Table 1). The eastern edge of the beach ridge plain is marked by a large hillslope deposit (Picture 1) over 6 m thick. A radiocarbon age at the base of this deposit indicates an age of 1181 to 1267 calibrated years before present.

The beach ridge plain at Ocean Beach records no evidence of inundation events or land-level changes in the last ~1000 years. Representative stratigraphy is presented in Figure 3.

4.2.2 Tombolo Marsh Tombolo Marsh records no evidence of inundation events or land-level changes in the last ~6000 years. The site is currently a salt marsh within the upper portion of the tidal range and is frequently inundated by high waters as marked by driftwood scattered prominently across the marsh.

Tombolo Marsh records no evidence of inundation events in the last ~6000 years. Interpretation of land-level changes is complicated by the uncertain relationship between the marsh and relative sea-level (the indicative meaning). However, the development of Bull Spit (section 4.2.3 below) would support the marsh having been in close proximity to sea level since at least 750 calibrated years before present. Representative stratigraphy is presented in Figure 4.

4.2.3 Bull Spit Bull Spit records no evidence of inundation events or land-level changes in the last ~750 years since it formed over beach deposits. Stratigraphy from core BSM.16.02 is presented in Figure 5.

4.2.4 Unga We described stratigraphy and collected cores at a raised peat bog (Unga Bog) above Unga Village to provide an age for the retreat of the Alaskan Peninsula ice sheet from the island. The radiocarbon age from the contact between basal peat and glacial till (Table 1) indicates ice withdrawal prior to 11200 calibrated years before present. This is slightly earlier than an age of 10400 calibrated years before present reported for Simeonof Island (Witter et al., 2014).

4.3 Geochemistry We collected three sediment samples from outcrop OB.16.11 for geochemical analyses for fecal sterols. All three sediment intervals from contained measurable amounts of fecal sterols (Figure 6). The middle depth interval 39-43 cm had the highest total concentration of 9.3 µg/g whereas the overlying sediments between 11-15 cm gave 6.3 µg/g and the underlying sediment interval at 57-64 cm yielded 5.6 µg/g. These total concentrations are lower than those observed in lake and river sediments receiving treated wastes and are far lower than that reported in the literature from undiluted fresh manures and human sewage that yield concentrations in 1000 -4000 µg/g (Vane et al., 2010). Nevertheless, the sterol concentrations presented herein are above the LOD and LOQ and are therefore fit for interpretation.

Coprostanol is the major 5b-stanol in human faeces comprising about 50 % of the total sterol content whereas the relative quantity of coprostanol in other animals is lower (Leeming et al., 1998). For example, in raw human waste the ratio of coprostanol to cholesterol is about 10 although it is known that this then decreases and then stabilises at coprostanol to cholesterol of about 2 after environmental attenuation. Faecal material from ruminants such as cows and sheep contain higher relative proportions of 5b-campestanol, 5b-stigmastanol and 5b-Sitsterol due to the high amount of precursor compounds in the vegetation they digest and subsequently excrete.

Examination of the distribution of fecal sterols (all intervals) suggests a very low amount of coprostanol and its environmental degradation product epicoprostanol which in turn suggests no human or for that matter carnivore (fox or dog) input (Figs 2 and 3). This evidence is confirmed by the very low coprostanol to cholesterol ratios of 0.8 (11-15cm) to 0.7 (39-43 cm) and 0.6 (57-64 cm).

In contrast, all three intervals gave high amounts of 5b-sitsterol, 5b-campestanol, 5b-stigmastanol suggesting a possible ruminant input. As discussed above it is reasonable to assume that the higher concentrations of 5b-sitsterol, 5b- campestanol, 5b-stigmastanol at the 39-43 cm interval reflects greater input (higher animal/density abundance) although these differences may also be attributed to varying amounts of mineral soil/sediment dilution. Potential increases in ruminant input suggest that this sedimentary boundary may be associated with the introduction of cattle to Unga in the 1830s (e.g., Reedy, 2016) that would be consistent with the radiocarbon ages at the base of the section.

5. Conclusions

1. We did not find evidence for land-level changes >0.3m on Unga Island.

2. We did not find evidence for marine inundation from tsunami or storms on Unga Island.

3. Fecal sterols are preserved in the sediments of Unga Island and show potential as an indicator of the presence of ruminants (cattle) that may be used to infer ages of sediments based on the introduction of cattle (e.g., Reedy, 2016).

4. Our findings lend additional support the work of Witter et al. (2014) from Simeonof Island who found no geological evidence of the 1788 CE earthquake and tsunami. These results are also supported by the lack of evidence for tsunami inundation in 1788 CE at Sanak Island (Engelhart et al., 2016).

5. While GPS suggests the megathrust offshore of the Shumagins is partially locked (e.g., Fournier and Freymueller, 2007; Li and Freymueller, 2018) we find no evidence of repeated land-level changes or marine inundation from tsunami at Unga Island. This is in addition to previous results from Simeonof Island that came to the same conclusion for the past ~4,500 years.

6. Bibliography of publications resulting from this award to date

Published abstracts:

Engelhart, S.E., Witter, R.C., Briggs, R.W., Dura, T., Koehler, R.D., Vane, C.H., Nelson, A.R., Gelfenbaum, G., Haeussler, P.J., 2018. Historical and stratigraphic evidence for two ruptures of the Alaska-Aleutian megathrust in 1788. European Geosciences Union General Assembly 2018, Vienna, Austria, EGU2018-9826.

7. References

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Carver, G.A., Plafker, G., 2008. Paleoseismicity and neotectonics of the Aleutian subduction zone: An overview, in Freymueller, J.T., Haeussler, P.J., and others (eds.), Geophysical Monograph Series: Washington D.C., American Geophysical Union, p. 43-63.

Ely, L.L., Cisternas, M, Wesson, R.W., and Dura, T., 2014. Five centuries of tsunamis and land-level changes in the overlapping rupture area of the 1960 and 2010 Chilean earthquakes. Geology 42, 995-998.

Engelhart, S.E., Horton, B.P., Dura, T., 2016. Paleoseismology of Sanak Island: Collaborative Research with University of Rhode Island, Rutgers University, and U.S. Geological Survey. USGS EHP report: https://earthquake.usgs.gov/cfusion/external_grants/reports/G14AP00081.pdf

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Kemp, A.C., Nelson, A.R., Horton, B.P., 2013b. Radiocarbon dating of plant macrofossils from salt-marsh sediments. Treatise on Geomophology.

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Li, S., Freymueller, J.T., 2018. Spatial variation of slip behavior beneath the Alaska Peninsula along Alaska-Aleutian subduction zone. Geophysical Research Letters, doi:10.1002/2017GL076761

Loveless, J.P., Meade, B.J., 2011. Spatial correlation of interseismic coupling and coseismic rupture extent of the 2011 Mw=9.0 Tohoku-Oki earthquake. Geophysical Research Letters 38, L17306.

Moreno, M., Rosenau, M., Oncken, O., 2010. 2010 Maule earthquake slip correlates with pre-seismic locking of Andean subduction zone. Nature 467, 198-202.

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Reedy, K., 2016. Kelp-fed beef, swimming caribou, feral reindeer, and their hunters: island mammals in a marine economy. Sustainability 8, doi:10.3390/su8020113

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Shennan, I., Brader, M.D., Barlow, N.L.M., Davies, F.P., Longley, C., Tunstall, N., 2018. Late Holocene paleoseismology of Shuyak Island, Alaska. Quaternary Science Reviews 201, 380-395.

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Vane, C.H., Kim, A.W., McGowan, S., Leng, M.J., Heaton, T.H.E., Kendrick., C.P., Coombs, P., Yang, H, Swann, G.E.A., 2010. Sedimentary records of sewage pollution using faecal markers in contrasting peri-urban shallow lakes. Science of the Total Environment 409, 345-356.

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Witter, R.C., Briggs, R.W., Engelhart, S.E., Gelfenbaum, G., Koehler, R.D., Barnhart, W.D., 2014. Little late Holocene strain accumulation and release on the Aleutian megathrust below the Shumagin Islands, Alaska. Geophysical Research Letters, doi:10.1002/2014GL059393

Witter, R.C., Carver, G.A., Briggs, R.W., Gelfenbaum, G., Koehler, R.D., La Selle, SP., Bender, A.M., Engelhart, S.E., Hemphill-Haley, E., Hill, T.D., 2016. Unusually large tsunamis frequent a currently creeping part of the Aleutian megathrust. Geophysical Research Letters, doi:10.1002/2015GL066083

Witter, R.C., Briggs, R.W., Engelhart, S.E., Gelfenbaum, G., Koehler, R.D., Nelson, A.R, LaSelle, SP., Corbett, D.R., Wallace, K., 2018. Evidence for frequent, large tsunamis spanning locked and creeping parts of the Aleutian megathrust. Geological Society of America Bulletin, doi:10.1130/B32031.1

Date + Site Position Sample NOSAMS ID Material Error Calibrated 13C Tombolo Marsh In Peat TM.16 3 21-22cm OS-130935 Stem 175 ± 20 0-285 -25.6 Tombolo Marsh Below Olive Tephra TMP 4B OS-130929 Woody stem 2160 ± 15 2115-2301 -25.8 Tombolo Marsh Above Keta Tephra TMP 4A OS-130930 Wood 3760 ± 20 4008-4227 -26.3 Tombolo Marsh In Keta Tephra TM 16 5 176-175.5cm OS-130934 Woody stem 3850 ± 20 4157-4405 -26.9 Tombolo Marsh Below Keta Tephra TM.16.OC6 111-111.5cm OS-134978 Woody stem 4090 ± 30 4448-4808 -27.5 13599- Tombolo Marsh Below Keta Tephra TMP 4C OS-130932 Woody stem 11850 ± 50 13766 -25.6 Tombolo Marsh Between Duchess and Keta Tephra TM 16 5 211-213cm OS-132265 Woody stem 4500 ± 20 5049-5289 -25.9 Tombolo Marsh 10cm above Duchess Tephra TM 16 2 158-160cm OS-132266 Wood 4970 ± 20 5649-5739 -24.7 Tombolo Marsh Above Duchess Tephra TMP 6C OS-130933 Stem 4920 ± 20 5598-5707 -28.2 Tombolo Marsh Below Duchess Tephra TMP 6A OS-130931 Large Stem 5280 ± 25 5947-6179 -28.2 Tombolo Marsh Above Green Tephra TM 16 OC6 204-205cm OS-134979 Wood 8380 ± 65 9149-9527 -28.4

Stems with 11271- Unga Bog Basal Peat UV 16 01 171-181cm OS-130812 nodes 10000 ± 45 11704 -28.4

Bull Spit Formation of spit minimum age BS 16 2 111cm OS-130813 Wood 900 ± 15 748-906 -26.5

Ocean Beach Base of soil on beach ridge OB 16 16 OS-132268 Bulk organics Modern Modern -27.4 Ocean Beach Basal sand in modern berm OB 16 6 OS-132269 Bulk organics 345 ± 15 317-478 -26 Ocean Beach Base of debris slide OB C1 OS-132267 Charred wood 1270 ± 15 1181-1267 -27.8 Ocean Beach Base of debris slide OB Bone 1 OS-131449 Whale bone 2970 ± 20 3071-3209 -18 Ocean Beach Organic sand in modern berm OB 16 2 OS-134980 Wood stems 940 ± 25 795-920 -27.2 Ocean Beach Organic sand in modern berm OB 16 8 OS-135067 Wood stems 770 ± 15 674-726 -26.2 Ocean Beach Organic sand in modern berm OB 16 11 OS-135068 Wood stems 1170 ± 20 1007-1176 -25.7

Table 1. AMS radiocarbon dates used in this study to provide a chronologic framework at Unga Island