On the tuning of plateaus in atmospheric and oceanic ¹⁴C records to derive calendar chronologies of deep-sea cores and records of ¹⁴C marine reservoir age changes

We assess the methodology of the so-called 14C plateau tuning (PT) technique used to date marine sediment records and determine 14C marine reservoir ages (MRAs) as recently reviewed by Sarnthein et al. (2020). The main identified problems are linked to the assumption of constant MRA during 14C age plateaus; the lack of consideration of foraminifera abundance changes coupled to bioturbation that can create spurious plateaus in marine sediments; the assumption that plateaus have the same shapes and durations in atmospheric and oceanic records; the implication that atmospheric 14C / 12C peaked instantaneously from one plateau to the next; that the 14C plateaus represent 82 % of the total time spent between 14 000 and 29 000 cal yr BP, whereas during the remaining 18 % of the time, the radiocarbon clock was running almost 5 times faster than the radioactive decay; that the sparsity, combined with the level of analytical uncertainties and additional noise, in both atmospheric and marine data do not currently allow one to reliably or robustly identify plateaus (should they exist) beyond 15 000 cal yr BP; and that the determination and identification of plateaus in the deep-sea cores is reliant upon significant changes in sedimentation rate within those marine sediments which are, a priori, unknown and are not verified with an independent method. The concerns we raise are supported and strengthened with carbon cycle box model experiments and statistical simulations of pseudo-atmospheric and pseudo-marine records, allowing us to question the ability to identify and tune 14C age plateaus in the context of noisy and sparse data.

The influence of lateral transport on sedimentary alkenone paleoproxy signals

Alkenone signatures preserved in marine sedimentary records are considered one of the most robust paleothermometers available, and are often used as a proxy for paleoproductivity. However, important gaps remain on the provenance and fate of alkenones, and their impact on derived environmental signals in marine sediments. Here, we analyze the abundance, distribution, and radiocarbon (14C) age of alkenones in bulk sediments and corresponding grain-size fractions in surficial sediments from seven continental margin settings in the Pacific and Atlantic Oceans in order to evaluate the impact of organo-mineral associations and hydrodynamic sorting on sedimentary alkenone signals. We find that alkenones preferentially reside within fine-grained mineral fractions of continental margin sediments, with the preponderance of alkenones residing within the fine silt fraction (2–10 µm), and most strongly influencing alkenone 14C age, and SST signals from bulk sediments as a consequence of their proportional abundance and higher degree of OM protection relative to other fractions. Our results demonstrate that selective association of alkenones with mineral surfaces and associated hydrodynamic mineral sorting processes can alter alkenone signals encoded in marine sediments (14C age, content, and distribution) and confound corresponding proxy records (productivity and SST) in the spatial and temporal domain.

Changes in carbonate state and other properties of fallow soils in the forest-steppe and steppe zones of Russia

<p>In the forest-steppe and steppe zones of Russia, soils are subject to prolonged agricultural impact which affected their properties and processes. Therefore, the study of soil transformation under different land-use regimes is an urgent task. The aim of the study is to examine the general patterns of changes in the carbonate state and other properties of soils of the steppe and forest-steppe in Russia of land-use changes from arable to abandoned land (fallow).</p><p>The objects of research are сhronosequences of fallow Chernozems and Phaeozems. The first сhronosequence is located in the Belgorod region, Russian Federation. It consists of a virgin, arable Phaeozems, and Phaeozems being in the fallow for 40-45 years. The second сhronosequence is located in the Rostov region. It consists of arable Chernozem and abandoned during 14, 20, 30, and 86 years Chernozems. The third сhronosequence is located in the Lipetsk region. It consists of arable Chernozems and abandoned during 15, 25 years Chernozems. The fourth сhronosequence is located in the Kursk region. It consists of a virgin, arable Chernozems and abandoned during 10, 25, and 50 years Chernozems.</p><p>It is noted that all soils in the abandoned land tend to restore virgin properties. Restoration of vegetation and water regime plays the main role in the acquisition of natural soil properties. For 25-30 years, the structural state is restored in Chernozems and Phaeozems. Overconsolidation of the subsurface horizon disappears in Chernozems in 10-15 years, and in Phaeozems it persists up to 40 years. The restoration of the organic carbon in Chernozems and Phaeozems proceeds in different ways. If in Сhernozems, in general, there is an increase in the content and reserves of organic carbon, then in Phaeozems, in the opposite, their decrease is observed. In the transition from arable to fallow soils, there is a decrease in the content and reserves of carbonate carbon due to a change in the water regime: the intensity of the ascending water flows decreases and descending - increases.</p><p>In fallow soils, the radiocarbon age of pedogenic carbonates decreases. In arable land "ancient" carbonates are pulled up closer to the day surface. And when plowing stops, they are gradually washed out into the depth of a profile. The greatest 14C age of carbonates is observed in fallow soils with large herbaceous vegetation, which sucks the moisture out from the depth with powerful roots.</p><p>The recovery time of the natural soil properties depends on the initial state of the soil, the intensity of the agrogenic impact, the use of soil-saving technologies under plowing, and fertilizers use. In general, Chernozems restore faster than Phaeozems. The carbonate state in all сhronosequences acquires the virgin (before plowing) features after about 30 years.</p>

Characterizing groundwater recharge dynamics and response times in large sedimentary basins using 14C age tracer data and a reactive transport modeling approach

<p>Large sedimentary basins represent major groundwater resources vital in sustaining terrestrial ecosystems and the various socio-economic activities essential to modern day society (drinking water supply, health, agriculture, energy, and industry). To address the ongoing and future impacts of climate change and anthropogenic activities on groundwater sustainability a better understanding of groundwater storage and flow dynamics in these multi-aquifer systems is crucial. Groundwater ages in this respect serve as an effective tool, providing valuable insight into rates and sources of groundwater recharge and subsurface heterogeneity. In this study we investigated the well-studied Aquitaine Basin located in Southwest France, the 2<sup>nd</sup> largest sedimentary basin in the country, with an extensive repository of hydrologic and geochemical data spanning several decades. A 3D regional numerical flow model was developed and extended to simulate reactive transport of radiogenic, <sup>14</sup>C. An inverse modeling approach using available <sup>14</sup>C activity data is implemented to infer groundwater ages and constrain modern and historic recharge sources and aquifer response times. Preliminary findings from spatial variations in <sup>14</sup>C concentrations and groundwater ages point to the role of aquitards as important sources of recharge in regional scale, multi-aquifer systems.</p>

Possible Sources of Salinity in the Upper Dibdibba Aquifer, Basrah, Iraq

Salinity increase in groundwater was investigated in the area between Al-Zubair and Safwan, and close to the Khor Al-Zubair Channel of southern Iraq. Thirty-nine groundwater samples from the shallow aquifer and one sample from the Khor Al-Zubair Channel were analyzed. The mean total dissolved solids are 7556 mg/L. The δ2H and δ18O plot in two groups are below the global meteoric water line. Group A indicates the evaporation effect of irrigation return-flow, while group B is characterized by depleted δ18O values due to recharge under colder climate. Deuterium excess values plot within the region of modern precipitation and dilution of groundwater by precipitating water. The groundwater residence time is between 1000 and 2000 years and combining 14C -age with SO42− shows a contrasting effect on groundwater on both sides of Khedr Almai Fault and the Zubair anticline, which indicates the role of these geological structures on the hydrochemical evolution in the western part. Jabal Sanam shows no clear effect in this regard. The ratio Cl−/Br− and sulfate in groundwater showed that the measured salinity in groundwater is the result of a mixing process between groundwater, seawater intruding from Khor Al-Zubair Channel, and water from septic tanks in addition to dry and wet sea spray, and irrigation return-flow.

On the tuning of plateaus in atmospheric and oceanic ¹⁴C records to derive calendar chronologies of deep-sea cores and records of ¹⁴C marine reservoir age changes

As an extended comment on the paper by Sarnthein et al. (2020), we express strong reservations about the methodology of the so-called 14C plateau tuning (PT) technique used to date marine sediment records and its implications on the determination of 14C marine reservoir ages (MRA). The main problems are linked to: the assumption of constant MRA during 14C-age plateaus; the lack of consideration of foraminifera abundance changes coupled to bioturbation that can create spurious plateaus in marine sediments; the assumption that plateaus have the same shapes and durations in atmospheric and oceanic records; the implication that atmospheric 14C/12C peaked instantaneously from one plateau to the next; that the 14C plateaus represent 82 % of the total time spent between 14,000 and 29,000 cal yr BP, whereas during the remaining 18 % of the time, the radiocarbon clock was running almost 5 times too fast; that the sparsity, combined with the level of analytical uncertainties and additional noise, in both atmospheric and marine data do not currently allow one to reliably or robustly identify plateaus (should they exist) beyond 15,000 cal yr BP; and that the determination and identification of plateaus is reliant upon significant changes in sedimentation rate within the marine sediments which are, a priori, unknown and are not verified with an independent method. The concerns we raise are supported and strengthened with carbon cycle box-model experiments and statistical simulations of pseudo-atmospheric and pseudo-marine records, allowing us to test the ability to identify and tune 14C-age plateaus, in the context of noisy and sparse data.

LONG-TERM CHANGES IN 14C AGE DIFFERENCES BETWEEN HUMIC ACID AND PLANT FRAGMENTS AND THEIR LINKS TO PAST CLIMATE CHANGE

ABSTRACT Radiocarbon (14C) dating has been widely used to determine the age of deposits, but there have been frequent reports of inconsistencies in age among different dating materials. In this study, we performed radiocarbon dating on a total of 33 samples from 8-m-long sediment cores recovered from the wetland of the Muljangori volcanic cone on Jeju Island, South Korea. Ten pairs of humic acid (HA) and plant fragments (PF) samples, and three pairs of HA and humin samples, from the same depths were compared in terms of age. The PF were consistently younger than the HA. Interestingly, the age difference between HA and PF samples showed a long-term change during the past 8000 years. To test whether there was an association between this long-term age difference and climate change, we compared with the carbon/nitrogen (C/N) ratios and total organic carbon isotope (δ13CTOC) values of the sediments, as indicators of the relative abundance of terrestrial and aquatic plants; these parameters showed similar long-term trends. This suggests that the increasing (decreasing) trend in age difference was influenced by long-term dry (wet) climate change.

A Top-to-Bottom Luminescence-Based Chronology for the Post-LGM Regression of a Great Basin Pluvial Lake

We applied luminescence dating to a suite of shorelines constructed by pluvial Lake Clover in northeastern Nevada, USA during the last glacial cycle. At its maximum extent, the lake covered 740 km2 with a mean depth of 16 m and a water volume of 13 km3. In the north-central sector of the lake basin, 10 obvious beach ridges extend from the highstand to the lowest shoreline over a horizontal distance of ~1.5 km, representing a lake area decrease of 35%. These ridges are primarily composed of sandy gravel and rise ~1.0 m above the alluvial fan surface on which they are superposed. Single grain luminescence dating of K-feldspar using the pIRIR SAR (post-infrared infrared single-aliquot regenerative dose) protocol, corroborated by SAR dating of quartz, indicates that the highstand shoreline was constructed ca. 16–17 ka during Heinrich Stadial I (Greenland Stadial 2, GS-2), matching 14C age control for this shoreline elsewhere in the basin. The lake regressed rapidly during the Bølling/Allerød (GI-1), before the rate of regression slowed during the Younger Dryas interval (GS-1). The lowest shoreline was constructed ca. 10 ka. Persistence of Lake Clover into the early Holocene may reflect enhanced monsoonal precipitation driven by the summer insolation maximum.