A Driftwood-Based Record of Arctic Sea Ice During the Last 500 Years From Northern Svalbard Reveals Sea Ice Dynamics in the Arctic Ocean and Arctic Peripheral Seas

We present a 500-year history of naturally felled driftwood incursion to northern Svalbard, directly reflecting regional sea ice conditions and Arctic Ocean circulation. Provenance and age determinations by dendrochronology and wood anatomy provide insights into Arctic Ocean currents and climatic conditions at a fine spatial resolution, as crossdating with reference chronologies from the circum-Arctic boreal forests enables determination of the watershed the driftwood originated from. Sample crossdating may result in a wide range of matches across the pan-boreal region, which may be biased toward regions covered by the reference chronologies. Our study considers alternate approaches to selecting probable origin sites, by weighting scores via reference chronology span and visualizing results through spatiotemporal density plots, as opposed to more basic ranking systems. As our samples come from naturally felled trees (not logged or both), the relative proportions of different provenances are used to infer past ocean current dominance. Our record indicates centennial-to decadal-scale shifts in source regions for driftwood incursion to Svalbard, aligning with Late Holocene high variability and high frequency shifts in the Transpolar Drift and Beaufort Gyre strengths and associated fluctuating climate conditions. Driftwood occurrence and provenance also track the northward ice formation shift in peripheral Arctic seas in the past century. A distinct decrease in driftwood incursion during the last 30 years matches the observed decline in pan-Arctic sea ice extent in recent decades. Our new approach successfully employs driftwood as a proxy for Arctic Ocean surface circulation and sea ice dynamics

Dropped in the ocean – 87Sr/86Sr as a provenance tool for ice-rafted Arctic driftwood

Provenance and age determinations of driftwood provide insights into Holocene Arctic Ocean surface currents and sea ice dynamics, with detailed reconstructions requiring a provenance methodology with fine temporal and spatial resolution. Determination of wood geographical provenance by genus data is limited by its spatially coarse resolution, while provenance by dendrochronological crossdating is reliant on available boreal forest reference chronologies which are abundant for recent centuries but more sparse for older periods of the Holocene. We present the development of novel techniques to refine the provenance of driftwood through radiogenic isotopic analysis (87Sr/86Sr). The use of geochemical techniques addresses limitations of current methods and opens the possibility of defining the role of atmospheric and oceanic circulation in sea ice and climatic changes throughout the Holocene at a finer spatial resolution than currently possible. This study investigates and develops geochemical 87Sr/86Sr fingerprinting of Arctic driftwood. To this end, it analyses driftwood samples from northern Svalbard and compares this technique with provenance regions obtained through dendrochronology, and with modelled and measured global 87Sr/86Sr reference databases. We conclude that the utilisation of 87Sr/86Sr ratios to establish provenance for Arctic driftwood has some potential, but identify important limitations in the method, concerning both the signal in samples and reference values required for provenance. Increased sample populations and source samples for calibration, as well as methodological improvements that address the likely overprinting linked to driftwood transport, are recommended to build upon this work. We conclude that at present dendro-provenancing continues to be the most powerful method to study sea ice dynamics from Arctic driftwood and suggest that given the high spatial granularity of 87Sr/86Sr across the boreal forest, this technique might only be usable in combination with previously developed provenance tools and reconstructions. This study provides a first step in the use of radiogenic isotopic analysis in a multi-proxy reconstruction of Holocene driftwood incursion onto high Arctic shorelines

Pb isotopes in individual sand-sized (>150 µm) ice-rafted feldspars deposited at subpolar North Atlantic Ocean Deep Sea Drilling Project Site 611 during the late Pliocene (Marine Isotope Stages, MIS, G6 through 100)

The onset of abundant ice-rafted debris (IRD) deposition in the Nordic Seas and subpolar North Atlantic Ocean 2.72 millions of years ago (Ma) is thought to record the Pliocene onset of major northern hemisphere glaciation (NHG) due to a synchronous advance of North American Laurentide, Scandinavian and Greenland ice-sheets to their marine calving margins during marine isotope stage (MIS) G6. Numerous marine and terrestrial records from the Nordic Seas region indicate that extensive ice sheets on Greenland and Scandinavia increased IRD inputs to these seas from 2.72 Ma. The timing of ice-sheet expansion on North America as tracked by IRD deposition in the subpolar North Atlantic Ocean, however, is less clear because both Europe and North America are potential sources for icebergs in this region. Moreover, cosmogenic-dating of terrestrial tills on North America indicate that the Laurentide Ice Sheet did not extend to ~39°N until 2.4 ±0.14 Ma, at least 180 ka after the onset of major IRD deposition at 2.72 Ma. To address this problem,we present the first detailed analysis of the geochemical provenance of individual sand-sized IRD deposited in the subpolar North Atlantic Ocean between MIS G6 and 100 (~2.72-2.52 Ma). IRD provenance is assessed using laser ablation lead (Pb) isotope analyses of single ice-rafted (>150 mm) feldspar grains. To track when an ice-rafting setting consistent with major NHG first occurred in the North Atlantic Ocean during the Pliocene intensification of NHG (iNHG), we investigate when the Pb-isotope composition (206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb) of feldspars deposited at DSDP Site 611 first resembles that determined for IRD deposited at this site during MIS 100, the oldest glacial for which there exists convincing evidence for widespread glaciation of North America. Whilst Quaternary-magnitude IRD fluxes exist at Site 611 during glacials from 2.72 Ma, we find that the provenance of this IRD is not constant. Instead, we find that the Pb isotope composition of IRD at our study site is not consistent with major NHG until MIS G2 (2.64 Ma). We hypothesise that IRD deposition in the North Atlantic Ocean prior to MIS G2 was dominated by iceberg calving from Greenland and Scandinavia. We further suggest that the grounding line of continental ice on Northeast America may not have extended onto the continental shelf and calved significant numbers of icebergs to the North Atlantic Ocean during glacials until 2.64 Ma

An alternative suggestion for the Pliocene onset of major northern hemisphere glaciation based on the geochemical provenance of North Atlantic Ocean ice-rafted debris

The onset of abundant ice-rafted debris (IRD) deposition in the Nordic Seas and subpolar North Atlantic Ocean 2.72 millions of years ago (Ma) is thought to record the Pliocene onset of major northern hemisphere glaciation (NHG) due to a synchronous advance of North American Laurentide, Scandinavian and Greenland ice-sheets to their marine calving margins during marine isotope stage (MIS) G6. Numerous marine and terrestrial records from the Nordic Seas region indicate that extensive ice sheets on Greenland and Scandinavia increased IRD inputs to these seas from 2.72 Ma. The timing of ice-sheet expansion on North America as tracked by IRD deposition in the subpolar North Atlantic Ocean, however, is less clear because both Europe and North America are potential sources for icebergs in this region. Moreover, cosmogenic-dating of terrestrial tills on North America indicate that the Laurentide Ice Sheet did not extend to ~39 ?N until 2.4 ± 0.14 Ma, at least 180 kyr after the onset of major IRD deposition at 2.72 Ma. To address this problem, we present the first detailed analysis of the geochemical provenance of individual sand- sized IRD deposited in the subpolar North Atlantic Ocean between MIS G6 to 100 (~2.72-2.52 Ma). IRD provenance is assessed using laser ablation lead (Pb) isotope analyses of single ice-rafted (>150?m) feldspar grains. To track when an ice-rafting setting consistent with major NHG first occurred in the North Atlantic Ocean during the Pliocene intensification of NHG (iNHG), we investigate when the Pb-isotope composition (206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb) of feldspars deposited at DSDP Site 611 first resembles that determined for IRD deposited at this site during MIS 100, the oldest glacial for which there exists convincing evidence for widespread glaciation of North America. Whilst Quaternary- magnitude IRD fluxes exist at Site 611 during glacials from 2.72 Ma, we find that the provenance of this IRD is not constant. Instead, we find that the Pb-isotope composition of IRD at our study site is not consistent with major NHG until MIS G2 (2.64 Ma). We hypothesise that IRD deposition in the North Atlantic Ocean prior to MIS G2 was dominated by iceberg calving from Greenland and Scandinavia. We further suggest that the grounding line of continental ice on Northeast America may not have extended onto the continental shelf and calved significant numbers of icebergs to the North Atlantic Ocean during glacials until 2.64 Ma

Surface Ocean CO2 Atlas (SOCAT) V6

The Surface Ocean CO2 Atlas (SOCAT) is a synthesis activity by the international marine carbon research community (>100 contributors). SOCAT version 6 has 23.4 million quality-controlled, surface ocean fCO2 (fugacity of carbon dioxide) observations from 1957 to 2017 for the global oceans and coastal seas. Calibrated sensor data are also available. Automation allows annual, public releases. SOCAT data is discoverable, accessible and citable. SOCAT enables quantification of the ocean carbon sink and ocean acidification and evaluation of ocean biogeochemical models. SOCAT represents a milestone in biogeochemical and climate research and in informing policy. 424 datasets Version 5: https://doi.pangaea.de/10.1594/PANGAEA.877863 Version 4: https://doi.pangaea.de/10.1594/PANGAEA.866856 Version 3: https://doi.pangaea.de/10.1594/PANGAEA.849770 Version 2: https://doi.org/10.1594/PANGAEA.81515