High-resolution analysis of the varved succession at Crawford lake across the base of the proposed Crawfordian stage and Anthropocene series

Abstract

Four years after the Anthropocene Working Group (AWG) voted to work toward defining the Anthropocene series/epoch with a base in the mid-20th C, the varved sediments of Crawford Lake (Milton, ON, Canada) were selected as the Global boundary Stratotype Section and Point (GSSP) candidate. The initial major rise in activity of ^{239 + 240}Pu had been selected as the primary chronostratigraphic marker to define the base of the Anthropocene, but the precise year when this occurred could not be determined from measurements of samples combining multiple varves. Individual varves from freeze cores collected in April 2023 provide annual resolution for bomb radionuclides, allowing the varve age model to be refined, former assignments determined to have been 1 year too old. The increase in ^{239 + 240}Pu activities (calculated from atom concentrations of ²³⁹Pu and ²⁴⁰Pu measured using Accelerated Mass Spectrometry) of 0.0031 Bq/g between varves now assigned to 1951 and 1952 is consistent with the onset of thermonuclear weapons testing on November 1, 1952, so the proposed base for the Anthropocene is at the contact between the light- and dark-coloured laminae deposited in 1952 CE (17.5 cm in core CRA23-BC-1F-B). Sharply lower ^{239 + 240}Pu and ¹³⁷Cs activities capture the moratorium from November 1958 to September 1961 before rising quickly to peak activities of ^{239 + 240}Pu in 1963 CE. Analysis of individual varves with varying amounts of organic matter and inorganic calcite illustrates the influence of lithology on organic proxies, but the upcore trend toward depleted values of δ¹⁵N through the 20th C reflects increased fossil fuel combustion worldwide. An inflection point in δ¹⁵N around 1911 CE is attributed the global impact of the Haber-Bosch process and establishment of nearby steel mills, and another in the early 1950s attributed to the Great Acceleration to which the tipping point in the Earth system is attributed.

Keywords: GSSP; Great Acceleration; radionuclides; stable isotopes; varves.

Figure 1.

Water below 15.5 m in Crawford Lake (the monimolimnion) is permanently isolated from the mixed layer above (the mixolimnion). Varved sediments accumulate undisturbed by physical or biological agents in the deep basin occupying the northern part of the small lake ~60 km west of Toronto, in the Lake Ontario catchment. Freeze cores were collected from the western part of the deep basin in April 2023. Dots show the location of core CRA23-BC-1F-B (black—the GSSP core proposed by AWG, 2024b) and CRA23- 2FT-A1 (red).

Figure 2.

Distinct varves can easily be correlated between the cross section (side-view) of core CRA22-1FR3 (imaged at ultra-high resolution; see Lafond et al., 2023) and the photograph of the recently collected face of core CRA23-BC-1F-A. Distinct varves characterise intervals of anthropogenic impact and cultural eutrophication (Indigenous agricultural settlement in the late 13th through 15th C and colonial impact beginning in the early 19th C), illustrating the difference between the many clear instances of local human impact through the Holocene and the concept of the proposed Anthropocene epoch which reflects the globally synchronous Earth System changes resulting from the Great Acceleration and marked by the first major increase in plutonium activity (AWG 2024b).

Figure 3.

Bomb radionuclide activities measured in an annual herbage archive from Rothamsted, UK, in cores from the Mont Blanc glacier, Milford Haven and Crawford Lake mirror the annual yields of atmospheric fallout from thermonuclear weapons testing (UNSCEAR, 2000). Note the uneven rise to the 1963–1964 peak in both ^{239 + 240}Pu and ¹³⁷Cs and clear response to the moratorium between late 1958 and 1961 CE. The earlier increase in radionuclides in varved sediments from Crawford Lake is attributed to proximity to atomic bomb testing in the SW USA. Modified from Warneke et al. (2002), adding data from freeze cores collected from Crawford Lake in 2019 and 2022 from McCarthy et al. (2023).

Figure 4.

The proposed GSSP is at 17.5 cm in the 89 cm-long freeze core CRA23-BC-1F-B, measured 12.4 cm from the left edge of the core face that is archived at the Canadian Museum of Nature. It is placed at the base of the dark lamina comprised primarily of authigenic organic matter deposited during turnover of the mixolimnion in fall of 1952 CE, and which overlies the thin calcite lamina deposited in the summer. This lithological boundary approximates (and is taken to represent) the moment when the first thermonuclear weapon (Ivy Mike) was detonated: 7:15 AM on November 1, 1952 MHT in the Marshall Islands. This varve is one of several with very thin summer laminae forming the lowermost of two dark bands that are easily correlated between cores across the deep basin of Crawford Lake. The thick calcite lamina at 20 cm, that serves as the starting point for the varve chronology upcore and downcore illustrated at right, is now assigned a calendar age of 1936 CE. Upper panel (left to right): core face, enlargement of core face, edge of core with image reversed to facilitate detailed correlation with the core face. Lower panel: enlarged image of the GSSP interval as indicated by the dashed red box in the upper panel.

Figure 5.

Gamma activity attributed to ¹³⁷Cs in varves from core CRA23-2FT-A is consistent with global fallout records, capturing the 1958–1961 moratorium as well as the decline following ratification of the Limited Test Ban Treaty in late 1963 CE (compare Figure 3). All analyses (spanning less than 5 cm in the core, illustrated here) are of individual varves except 1953–1954 CE, which were combined for analysis, and thus illustrated by an ellipse rather than a circle.

Figure 6.

Plutonium isotopes across the proposed base of the Anthropocene (green line, base of the calcite lamina in the varve deposited in 1952 CE) in varved sediments from Crawford Lake. (a) Blank-corrected concentrations of atoms of ²³⁹Pu per dry weight measured using accelerator mass spectrometry (AMS) in 18 individual varves from freeze cores collected in 2023, activities of ²³⁹Pu and ²⁴⁰Pu calculated from the atomic concentrations, and ratios of ^240/239Pu (Table 4). (b) Plutonium activities at annual resolution plotted with data from freeze cores collected in 2019 and 2022 (red and green symbols), all subsampled with reference to the same thick, distinctive marker varve visible at lower right in the photograph of the proposed GSSP core CRA23- BC-1F-B. (c) Plutonium activities are clearly consistent with global yields, increasing steadily to 1954 CE from very low activities in 1951 CE, and rising quickly from 1961 to the peak in 1963 CE, more clearly identified with higher resolution sampling of individual varves. The data therefore record both the upturn of Pu activities following the onset of high-yield thermonuclear weapons testing in 1952, and earlier testing activity. The low activities measured in 1961 CE confirm the shoulder in the profile that identifies the 1958–1961 moratorium.

Figure 7.

Carbon and nitrogen records for core CRA23-2FT-A1. Generally, C and N covary in endogenic varves composed almost entirely of authigenic calcite capping organic matter derived primarily from algae and their consumers, so C:N ratios tend to hover around the mean until the mid-20th C, when very high C:N ratios and less depleted values of δ13C were measured in varves with prominent calcite laminae. Vertical lines illustrate mean values of organic proxies in individual varves during the late 19th C (dark grey shading), early 20th C (light grey) and late 20th C (no shading). Stable isotopes were most depleted in dark calcite-poor, highly organic sediments (mean %C = 17.9%, %N = 1.8%) deposited in the late 19th C, when a lumber mill operated on the south shore of the lake. The proposed Anthropocene epoch (above the green line, 1952 CE) is characterised by depleted values of δ15N (mean ~1.3 ‰ lower than during the first half of the 20th C), although the overall trend is toward increasingly depleted values upcore since 1912 CE.

Figure 8.

Results of ITRAX micro-XRF analysis of varved sediments from Crawford Lake. Peaks in Ti:Ca are attributed to increased terrigenous sediment influx due to land disturbance, soil erosion and/ or increased precipitation and runoff from the small catchment. Image of core CL-2011 superimposed on the RGB core scan with varve age model from this article. Heavy metals (Cu, Pb, Zn) associated with fly ash are more abundant in sediments deposited between 1952 CE (green line marking the proposed base of the Anthropocene) and 1980 CE and are reduced in calcite-rich (light coloured) laminae as an artifact of the strong contribution of Ca to total elemental abundances.

Figure 9.

(a) Measurements of δ¹⁵N on individual varves from core CRA23-2FT-A1 show a gradual decline (dashed line), particularly beginning in the second decade of the 20th C, illustrating the global effects of mass-produced automobiles and nitrogen fixing using the Haber-Bosch method, and the regional effects of steel mills. The red line shows a similar trend measured in samples combining multiple varves, published in McCarthy et al. (2023) but since corrected for a spreadsheet error at U Leicester. The lower δ15N mean through the proposed Anthropocene (4.46‰) to the post-1952 CE mean (5.15‰, n = 100). (b) High concentrations of fly ash in freeze cores CRA19-2FT-B2 (red) and CRA22-1FR-A (black) reflect the rapid increase in high temperature combustion of fossil fuels during the Great Acceleration, particularly at steel mills in nearby Hamilton where half of Canada’s steel was produced during the post-WWII boom, although higher relative abundances of smaller particles since record more distal influence in the latter half of the 20th C.