Extreme decay of meteoric beryllium-10 as a proxy for persistent aridity

The modern Antarctic Dry Valleys are locked in a hyper-arid, polar climate that enables the East Antarctic Ice Sheet (EAIS) to remain stable, frozen to underlying bedrock. The duration of these dry, cold conditions is a critical prerequisite when modeling the long-term mass balance of the EAIS during past warm climates and is best examined using terrestrial paleoclimatic proxies. Unfortunately, deposits containing such proxies are extremely rare and often difficult to date. Here, we apply a unique dating approach to tundra deposits using concentrations of meteoric beryllium-10 (10Be) adhered to paleolake sediments from the Friis Hills, central Dry Valleys. We show that lake sediments were emplaced between 14–17.5 My and have remained untouched by meteoric waters since that time. Our results support the notion that the onset of Dry Valleys aridification occurred ~14 My, precluding the possibility of EAIS collapse during Pliocene warming events. Lake fossils indicate that >14 My ago the Dry Valleys hosted a moist tundra that flourished in elevated atmospheric CO2 (>400 ppm). Thus, Dry Valleys tundra deposits record regional climatic transitions that affect EAIS mass balance, and, in a global paleoclimatic context, these deposits demonstrate how warming induced by 400 ppm CO2 manifests at high latitudes.

Scientific RepoRts | 5:17813 | DOI: 10.1038/srep17813 We present a rare, continuous record of climate change contained within the innermost, highest elevation zone of the Dry Valleys. The Friis Hills, Taylor Valley (800 m above sea level) contains a thick (14 m) series of stacked glacial drifts found interbedded with silty paleolacustrine sediments. These sediments contain a diverse fossil assemblage now extinct in Antarctica including Nothofagus (southern beech) wood and leaves 13 . Although brine lakes commonly exist alongside and under Antarctic glaciers under the modern climatic regime 14 , the fossils within Paleolake Friis sediments were likely deposited in a semi-permanent proglacial lake on wet, freshwater tundra. Because modern climatic conditions at the Friis Hills are extremely cold (average annual temperature: − 22 °C) and arid (lows measured < 16% relative humidity) 15 , these deposits must archive a period of warmer and wetter climatic conditions. Directly dating these sediments becomes necessary to resolve when tundra-like conditions last prevailed in the upper, inner Dry Valleys.

Meteoric beryllium-10 as an age indicator
To provide chronologic control for the lake sediments we utilize beryllium-10 ( 10 Be) as an isotopic tracer. Cosmic-ray-produced (cosmogenic) 10 Be forms in the atmosphere when high-energy neutrons from secondary cosmic rays spall nitrogen and oxygen atoms. This 10 Be, denoted meteoric 10 Be, exists in the form of 10 BeO and 10 Be(OH) 2 in the atmosphere and quickly adheres to atmospheric aerosols (primarily sulfates) 16 . The 10 Be-bearing aerosols are then delivered to the Earth's surface through wet (rain) or dry (dust) deposition. Through continued deposition, meteoric 10 Be will accumulate at the surface and at depth, as 10 Be moves into the soil column via infiltration and clay illuviation 17 .

Sediment age model
Concentrations of meteoric 10 Be adhered to Paleolake Friis sediments are used to model a minimum age of paleolacustrine deposition. Lebatard et al. (ref. 17 first demonstrated that it is possible to date ancient terrestrial deposits with meteoric 10 Be if, once buried, sediments remain a closed system. One way to achieve this prerequisite is if meteoric waters do not infiltrate the subsurface. When these conditions are met, the measured 10 Be reflects the initial inventory that was present at the time of burial, [ 10 Be] initial , which is only altered by decay. A hyper-arid climate in the Dry Valleys provides the conditions needed for a closed 10 Be system, allowing the use of meteoric 10 Be as a chronometer. To model sediment age, we first determine a range of potential [ 10 Be] initial . This is possible if we model lake sediments as soil surface sediments that have reached equilibrium between 10 Be gain (via deposition) and loss (via erosion and decay). Solving Willenbring and von Blanckenburg's equation for steady state erosion rate (ref. 16, Eq. 21) we estimate a likely range of [ 10 Be] initial that was accumulated before burial: where Q is flux of 10 Be to the Earth's surface (atoms cm −2 y −1 ), ρ is soil density (1.57 g cm −3 ) and E is erosion rate (cm y −1 ). We use the 10 Be flux calculated for Table Mountain (3.4 × 10 3 atoms cm −2 y −1 ) 18 , a nearby location that is a suitable representative analog of Friis Hills because comparable arid, windy conditions disallow accumulation of atmospheric aerosols on the earth's surface. To estimate E, we use a range of plausible erosion and total denudation rates obtained independently throughout the Dry Valleys on bedrock and regolith material (Supplementary Table S1). Once the lake sediments were buried, the [ 10 Be] initial began to decay to their current concentration. To determine how long this took, we solve the radioactive decay equation for time, t: where N(t) is the measured [ 10 Be] in buried lake sediments (atoms g −1 ), N 0 is [ 10 Be] initial (atoms g −1 ) determined using Eq. 1, and λ is the 10   Central to our approach are measurement capabilities. The 10 Be concentrations are measured using an accelerator mass spectrometer (AMS); the detection sensitivity is ~10 4 atoms g −1 . Given the half-life of 10 Be (t 1/2 = 1.387 My) 19 , the detection limit corresponds to a maximum age of ~14 My. That is, assuming no [ 10 Be] in the buried lake sediment is lost to erosion, an AMS measurement of [ 10 Be] within error of ~10 4 atoms g −1 indicates a lake sediment age of at least 14 My.

Results
Three samples collected at or below 26 cm depth at the Friis Hills fall below or within the 1-σ uncertainty of "blank" samples (Table 1). These measured concentrations approach the analytical limit of AMS ( 10 Be/ 9 Be ≈ 9 × 10 −16 ) and chemical extraction process (ranging from 10 Be/ 9 Be ≈ 1 × 10 −15 to 5 × 10 −15 ). We note that other publications measure concentrations at the surface and at depth up to two and six orders of magnitude greater, respectively 19,21-23 ; see Fig. 1. Higher concentrations may simply be a reflection of younger surfaces. The most comparable measurements made elsewhere are from Table Mountain Scientific RepoRts | 5:17813 | DOI: 10.1038/srep17813 ( Fig. 1, profile TM4). These data have been corrected for contamination from in situ 10 Be. While meteoric 10 Be is adsorbed to the outside of clay minerals, in situ 10 Be is produced and contained within the mineral structure itself. As Dickinson et al. (ref. 20 note, in situ concentrations are commonly < 1% of the meteoric 10 Be concentrations, but because of the great age of Dry Valleys sediments these two fractions may be of the same magnitude. In situ 10 Be concentrations are most likely liberated via partial decomposition of the clay mineral due to an aggressive leaching solution. By correcting for this contamination, the authors constrain the 10 Be flux value (Q) that we use to model the [ 10 Be] initial range.
The modeled [ 10 Be] initial range is 0.83 to 22 × 10 7 atoms g −1 (Supplementary Table S1). We compile a database of [ 10 Be] measurements from modern and ancient lake sediments and find that our estimates are well within the range of published values (Supplementary Table S2). To determine when buried lake sediments were emplaced, we solve Eq. 2 using this [ 10 Be] initial range and N(t) = 3.48 × 10 4 ± 3.48 × 10 4 atoms g −1 , the concentration of the chemical blank used to represent buried lake sediments, to produce a range of 11.0-17.5 My; see Fig. 2. Based on AMS measurement capabilities, lake sediments containing [ 10 Be] within error of the chemical blank are at least 14 My; see above. Accordingly, we raise the lower age limit from 11 My to 14 My. The upper limit of 17.5 My is in agreement with a 19.76 ± 0.11 My ( 40 Ar/ 39 Ar dated) ash that lies stratigraphically below sampling Pit 1 to the east 23 26 and leaf wax abundance 27 studies. These studies recognize periods of a retracted EAIS margin, decreased sea ice coverage, increased precipitation along the Ross Sea coastline, and a proliferation of vegetation. Definitive terrestrial evidence of the MMCO is found in high altitude tills deposited by wet-based ice 28 23 . A synchronized transition to arid conditions is recorded in volcanic ashes in nearby Olympus 28 and western Asgard Ranges 5,6 . Workers note that ashfalls infill sand-wedge troughs, which form only in cold/dry conditions, contain glass shards, and lack evidence of cryoturbation or clay formation. This pristine preservation indicates no presence of surface moisture or chemical weathering since the time of ash emplacement. The oldest, unaltered ash deposits in the Dry Valleys indicate that other parts of the region have experienced uninterrupted polar desert climate since ~15 My 5 ; our results expand this zone.
Based on the abundant evidence for warmer global and regional temperatures ~15-17 My, we suggest that Paleolake Friis sediments were likely emplaced during the MMCO (Fig. 3). At their warmest, terrestrial summer temperatures reached as high as 10 °C 26 , great enough to support a wet tundra environment in which fossils like Nothofagus thrived [ref. 4 associated this species with mean summer temperatures of ~5 °C]. Warmer temperatures coincide with increases in global CO 2 reconstructions. According to Royer's data compilation 9 the MMCO is arguably the last time global CO 2 remained > 400 ppm for several million years, making CO 2 a possible driver of EAIS retraction and terrestrial plant proliferation at this time. The linkages between global CO 2 concentrations, ice volume and vegetation during the MMCO have proven challenging to model, but these simulations are valuable towards our understanding of future climate and require improvement. A notable model deficiency is the lack of reliable temperature proxy data, particularly at high latitudes (e.g. ref. 32). The Paleolake Friis deposits, along with those described in Lewis et al. (ref. 8), represent the southernmost terrestrial deposits, and highest latitude deposits overall, available for middle Miocene paleoclimatic reconstructions. These records should be incorporated as constraints when modeling the MMCO; they are especially useful in reconstructing Equator to pole temperature gradients.
Following their deposition, Paleolake Friis sediments entered a closed system, one that did not receive meteoric 10 Be in surface waters via ice melt or precipitation. This closed system is maintained if plunging temperatures of the MMCT ~14 My were accompanied by an onset of extreme aridity. The lack of 10 Be in lake sediments indicates persistent polar aridity was established in the inner Dry Valleys by at least this time, contradicting the notion of large-scale EAIS collapse during the Pliocene. Methods Treating paleolake sediments. During the austral summer of 2008, five samples for meteoric 10 Be dating were collected from silty paleolacustrine sediments exposed on a hillside within the Friis Hills stacked tills. Samples were prepared at the University of Pennsylvania Cosmogenic Isotope Lab following protocol for adhered meteoric 10 Be extraction, including a 0.5 M HCl agitated leach and a 1 M hydroxylamine hydrochloride (NH 2 OH•HCl) leach in an ultrasonic bath 33 . Following 9 Be spike addition (GFZ German Research Centre for Geosciences "Phenakite" standard, 10 Be/ 9 Be spike = 10 −16 ) and ion exchange chromatography, samples were oxidized over open flame, packed with Nb powder into cathode targets, and sent to the Purdue PRIME Lab for AMS measurement of 10 Be/ 9 Be. Error assessment. The overall range of erosion rates reported in the literature is 0.1-2.6 m My −1 corresponding to an overall range of [ 10 Be] initial of 0.83-22 × 10 7 atoms g −1 . Not all published erosion rates are reported with associated errors and cannot be recalculated because in most cases erosion rate error distributions, 9 Be carrier spike, and/or assumed 10 Be/ 9 Be spike ratio were not reported. These missing data prohibit the inclusion of simple error propagation in our age model. Nevertheless, to assess the impact of error on our age estimates we apply a commonly reported error value of 10% to the upper and lower erosion rate estimates. This implementation results in marginally different lake sediment age estimates (10.7-17.7 My). Thus, incorporating erosion rate error does not affect our overall thesis that sediments were emplaced during the MMCO.
Choosing a flux value, Q. The proper choice of a flux value, Q, is critical for constraining the emplacement age of Friis Hills sediments. We have chosen a relatively low flux value of 3.4 × 10 3 atoms cm −2 y −1 because it was quantified from a nearby location with extremely similar climatic and erosional  24 . Grey shading: global atmospheric CO 2 (ppm) reconstruction, 5 point smooth 9 . Green line: Nothofagidites (type Nothofagus fusca) pollen abundance (count per grams dry weight, gdw −1 ) measured in AND-2A core 27 . The pollen abundance peak ~16.4 My is further evidence that Nothofagus, those leaf fossils in Paleolake Friis sediments, existed on Antarctica during the MMCO. The dotted green line represents a sedimentary hiatus in the AND-2A core that is attributed to ice sheet growth. conditions 19 . Other applications of meteoric 10 Be dating in the Dry Valleys have instead used a higher flux value that was determined from 10 Be accumulated in the Taylor Dome ice core (1.3 × 10 5 atoms g −1 y −1 ) 34 . Using this greater flux value for Paleolake Friis sediments yields a much older emplacement age of 18. 2-24.8 My. This range exceeds the underlying ash's age of 19.76 My 12 . As such, we regard the Taylor Dome flux value as unreasonably high for the Friis Hills location.