Conference Agenda

Our young Earth operated under conditions fundamentally distinct from today, with reduced solar luminosity, elevated CO₂, intense UV radiation, and acidic, low‑pH oceans. Sustained by strong greenhouse forcing, atmospheric CO₂ declined from Hadean levels to progressively lower Archaean–Proterozoic concentrations, coinciding with the advent and diversification of prokaryotic life by ~3.8 Ga, or possibly earlier. This keynote explores how coupled ocean–crust–atmosphere processes governed early ocean chemistry and sustained long‑term habitability. Palaeoarchaean records reveal dominantly submarine volcano‑sedimentary successions shaped by pervasive hydrothermal alteration and dynamic biogeochemical cycling. Silicification and carbonatization served as key alteration pathways, with near‑seafloor rocks crosscut by hydrothermal, chert‑filled fractures and overlain by bedded cherts that seal alteration zones and preserve carbonaceous matter‒potential archives of early life. Volcanic massive sulfide deposits and altered oceanic crust further attest to nutrient‑rich, hydrothermally driven ecosystems. Before significant continental emergence (~3.0 Ga), submarine ecosystems were largely sustained by nutrients derived from anoxic alteration of komatiitic and basaltic crust. The subsequent expansion of continental landmasses, the onset of oxygen oases, and development of shallow‑marine platforms during the Meso‑ to Neoarchean initiated fundamental shifts in seawater chemistry and biosphere structure, possibly punctuated by early icehouse conditions. By integrating field‑based, sedimentological, geochemical, and alteration signatures, this keynote highlights the co‑evolution of ocean chemistry and Earth‑system processes across deep time, emphasizing feedbacks among tectonics, hydrothermal fluxes, and the early biosphere, and constraining the limits of planetary habitability.

Reconstructing ancient seawater chemistry is fundamental to deciphering early Earth processes, which are often difficult to trace. Iron formations, marine chemical sediments characteristic of Precambrian oceans, are regarded as tracers of oceanic chemistry. We present a boron isotope record from Archean–Proterozoic marine deposits (cherts, iron formations, and shales) as a novel proxy for tracing changes in seawater composition through deep time.

Boron is enriched in continental crust relative to the mantle, making ocean boron concentration and isotopic composition (δ¹¹B) highly sensitive to the balance between continental weathering and seafloor hydrothermal alteration. Continental runoff and hydrothermal alteration represent the largest boron source and sink for the oceans respectively, linking seawater δ¹¹B directly to the hydrosphere–lithosphere interface and surface weathering dynamics.

Our comprehensive marine δ¹¹B record reveals a pronounced compositional shift at 3.0 Ga. Pre-3.0 Ga deposits yield mean δ¹¹B values of −23.1 ± 2.7‰, while post-3.0 Ga sediments are more variable, with a mean of −8.9 ± 3.1‰, representing seawater δ¹¹B = +16‰ by 2.4 Ga. This shift reflects a substantial increase in continental-derived boron flux to the oceans, driven by enhanced subaerial erosion following craton emergence after 3.0 Ga. Onset of subduction tectonics and modern seafloor spreading established present-day patterns of hydrothermal crustal alteration, which, together with plant-driven incongruent terrestrial weathering, ultimately pushed the ocean toward its modern equilibrium value (δ¹¹B = +39.6‰). Our results demonstrate that boron isotopes in marine sediments record secular changes in ocean–continent interaction and Earth-system coupling across critical intervals of Earth history.

The sedimentary rocks of the Pretoria Group preserve a nearly continuous record from the onset of the Great Oxidation Event (GOE) to the positive carbon-isotope excursion of the Lomagundi Event (LE). While its lowermost formations have been extensively studied for atmospheric and oceanic redox conditions, the links between rising atmospheric oxygen, climate evolution, and continental weathering throughout the entire sedimentary succession remain poorly constrained.

Here, we present new major and trace element data at high-spatial resolution from multiple drill cores spanning this stratigraphic interval, integrating the aforementioned links with sedimentological observations. The Rooihoogte and lower Timeball Hill (TBH) formations indicate intense continental weathering under a warm and humid climate following the GOE. A pronounced decline in weathering intensity in the upper TBH Formation suggests climatic cooling, culminating in deposition of the Rietfonteindam Diamictite, reflecting a low-latitude glaciation event. Weathering intensity increases again within the overlying Boshoek Formation, indicating renewed warming that continues into the red beds of the Dwaalheuwel Formation, suggesting extreme weathering under potentially hothouse conditions. A subsequent decrease in weathering intensity across the Strubenskop and Silverton formations, corresponding to the LE, suggests a cooling trend associated with elevated atmospheric O₂ levels, although not linked to a major glaciation. Increasing weathering intensity in the upper Silverton and Magaliesberg formations suggests renewed warming potentially linked to declining atmospheric oxygen during the later stages of the LE. Our results highlight the dynamic coupling between climate and continental weathering during the rise and consolidation of atmospheric oxygen between ~2.3 and 2.0 Ga.

The Nuup Kangerlua (Godhabsfjord), located in southwestern Greenland near the capital Nuuk, is the largest fjord area on the Labrador Sea. Its catchment is almost exclusively composed of Archean supracrustal bedrock (gneisses, granites, metavolcanics). This unique geological setting allows to study water-rock interaction and transport of solutes from Archean rocks to the ocean under Arctic conditions. We report rare earth element and yttrium (REY) concentration data for water samples from different fjords, creeks/rivers, lakes, and icebergs, sampled in June 2022. We show that the dissolved REY load (0.2µm-filtered) is dominated by continental run-off affected by water-rock interaction with Archean bedrock. We report fjord water samples at up to seawater salinities, that show a strong influence of nanoparticles and colloids (NPCs) and, as a result, REY anomalies (La_SN, Ce_SN, Gd_SN, Y_SN) and the typical W-type tetrad effects of seawater, but also REY features unique to Arctic freshwater (LREY_SN to HREY_SN enrichment). Like the investigated fluvial and limnic samples, some fjord waters show negative Eu_SN anomalies, most likely resulting from weathering of Archean bedrock. This is further supported by short-term low-pH leaching experiments of local bedrock and sediment. These results suggest that fjords with an Archean hinterland show a REY signature that is markedly different from fjords with a Paleozoic hinterland (e.g., Norway). This signature is evident even at high seawater-to-freshwater mixing ratios (33.5 PSU) and may thus serve as proxy for water-mass mixing and solute sources in seawater of the Davis Strait and, like Nd isotope ratios, as water mass tracer.