Ever since it was first proposed that the Earth completely froze during glaciations ∼ 640 million years ago evidence supporting this hypothesis has been increasing, primarily from samples of carbonates directly overlying glacial diamictites, termed cap carbonates. However, this was not the first extensive glacial period that affected planet Earth: ∼1750 million years prior to Neoproterozoic glaciations the Earth went through its first major glacial episode, the early Paleoproterozoic Huronian glaciations. The second Huronian ice advance deposited the Bruce Formation, with its overlying cap carbonate, the Espanola Formation. This up to ∼ 300 m thick succession of limestone, siltstone, dolostone and sandstone overlies diamictite containing a dropstone-bearing layer with δ¹³C_carb of −10‰. The ¹²C-enriched interval also has rare earth element (REE) patterns with negative Eu anomalies, radiogenic Sr isotopes, and negative εNd(0) in the carbonate. The first of these observations is probably due to highly reducing conditions in the sediment, and the possible thawing of methane-rich areas, releasing fluids that mixed with the overlying bottom waters; the last two reflect the diagenetic incorporation into the carbonate of radiogenic Sr, and derivation of REEs, including Nd, from abundant silty loess. This infers a stratified water mass with a relatively stagnant bottom layer during disintegration of an ice shelf. Above this REE patterns through the basal Espanola become increasingly more light depleted upwards, C becomes heavier, Sr is less radiogenic, εNd(0) is near 0 and one area has up to ∼ 1300 ppm Ba incorporated into the carbonate, indicating breakdown of water-mass stratification. Vertically over ∼ 200 m δ¹³C_carb increases from −4.5 to −2.5‰ as the environment shallowed incorporating gradually increasing amounts of seawater into the freshwater plume, which initially extended to depths below wave base. Strata deposited in the upper Espanola near the strandline contain layers of Fe-Mn-rich dolomite with positive Eu anomalies reflecting Paleoproterozioc seawater composition dominating even the nearshore by this time. These observations are similar to those from Neoproterozoic cap carbonates, and provide new evidence for the possibly snowball Earth-like nature of the ∼ 2.4 Ga Bruce glaciation.

自从大约 6.4 亿年前首次提出地球在冰川期间完全冻结以来，支持这一假设的证据一直在增加，主要来自直接覆盖在冰川混叠岩上的碳酸盐样品，称为帽状碳酸盐。然而，这并不是影响地球的第一个广泛的冰川期：在新元古代冰川之前约 17.5 亿年，地球经历了它的第一个主要冰川期，即早期古元古代休伦期冰川。第二次休伦冰层推进沉积了布鲁斯组及其上覆的碳酸盐岩，即埃斯帕诺拉组。这层高达 ~ 300 m 厚的石灰岩、粉砂岩、白云岩和砂岩覆盖在混叠岩上，该混叠岩包含一个含 δ ¹³ C_碳的滴石层-10‰。在¹²C 富集层段还具有稀土元素 (REE) 模式，具有负 Eu 异常、放射性 Sr 同位素和碳酸盐中的负 εNd(0)。这些观察中的第一个可能是由于沉积物的高度还原条件，以及富含甲烷的区域可能融化，释放出与上覆底水混合的流体；最后两个反映了成岩作用并入了放射性 Sr 的碳酸盐，以及从丰富的粉质黄土中衍生出包括 Nd 在内的稀土元素。这推断在冰架解体期间，分层水团具有相对停滞的底层。在通过基底 Espanola 的该 REE 模式之上，向上耗尽的光越来越多，C 变得更重，Sr 的放射源性降低，εNd(0) 接近 0，并且一个区域的碳酸盐中含有高达 1300 ppm 的 Ba，表明水团分层的分解。垂直超过 ∼ 200 m δ¹³ C _carb从-4.5 增加到-2.5‰，因为环境变浅，海水逐渐增加到淡水羽流中，淡水羽流最初延伸到波基以下的深度。沉积在上埃斯帕诺拉海岸线附近的地层包含富含铁锰的白云岩层，其中 Eu 正异常反映了古元古代海水组成，此时甚至在近岸占主导地位。这些观察结果与新元古代盖层碳酸盐的观察结果相似，并为 ∼ 2.4 Ga Bruce 冰川可能具有类似滚雪球的地球性质提供了新的证据。