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Fjord network in Namibia: A snapshot into the dynamics of the late Paleozoic glaciation
Geology ( IF 5.8 ) Pub Date : 2021-12-01 , DOI: 10.1130/g49067.1
Pierre Dietrich 1, 2 , Neil P. Griffis 3, 4 , Daniel P. Le Heron 5 , Isabel P. Montañez 3 , Christoph Kettler 5 , Cécile Robin 1 , François Guillocheau 1
Affiliation  

Fjords are glacially carved estuaries that profoundly influence ice-sheet stability by draining and ablating ice. Although abundant on modern high-latitude continental shelves, fjord-network morphologies have never been identified in Earth's pre-Cenozoic glacial epochs, hindering our ability to constrain ancient ice-sheet dynamics. We show that U-shaped valleys in northwestern Namibia cut during the late Paleozoic ice age (LPIA, ca. 300 Ma), Earth's penultimate icehouse, represent intact fjord-network morphologies. This preserved glacial morphology and its sedimentary fill permit a reconstruction of paleo-ice thicknesses, glacial dynamics, and resulting glacio-isostatic adjustment. Glaciation in this region was initially characterized by an acme phase, which saw an extensive ice sheet (1.7 km thick) covering the region, followed by a waning phase characterized by 100-m-thick, topographically constrained outlet glaciers that shrank, leading to glacial demise. Our findings demonstrate that both a large ice sheet and highland glaciers existed over northwestern Namibia at different times during the LPIA. The fjords likely played a pivotal role in glacier dynamics and climate regulation, serving as hotspots for organic carbon sequestration. Aside from the present-day arid climate, northwestern Namibia exhibits a geomorphology virtually unchanged since the LPIA, permitting unique insight into this icehouse.Fjords are long, deep, and narrow glacially carved estuaries that were occupied by outlet glaciers. They play a dramatic role in ice-sheet stability (i.e., drainage and ablation), are sensitive to climate change during icehouse periods, and act as important sediment sinks (Syvitski et al., 1987; Bennett, 2003; Kessler et al., 2008; Briner et al., 2009; Moon et al., 2018). Despite occupying just 0.1% of Earth's modern oceans, fjords account for >10% of Earth's organic carbon burial and may thus significantly impact the global carbon cycle (Smith et al., 2015). Fjord morphology and the sedimentary infill moreover play an instrumental role in assessing ice-sheet dynamics and climate change (Eilertsen et al., 2011; Steer et al., 2012; Normandeau et al., 2019; Bianchi et al., 2020). Despite their present-day abundance across high-latitude continental shelves, fjords from pre-Cenozoic glacial epochs have mostly been inferred from stratigraphic relationships and geological mapping (e.g., Kneller et al., 2004; Tedesco et al., 2016, and references therein), while the existence of paleo-fjord networks have not been rigorously established. The scarcity of ancient fjord morphologies seemingly reflects the intrinsic transient nature of these large-scale geomorphological features (Bianchi et al., 2020), rendering them prone to erosion over geological time scales. In turn, our ability to fully embrace large-scale dynamics of deep-time ice sheets and to assess their sensitivity to climate change is hindered.We present a geomorphic and sedimentologic analysis of a pristine paleo-fjord network across northwestern Namibia, which formed during Earths’ penultimate and long-lived icehouse, the late Paleozoic ice age (LPIA, 360–260 Ma; Montañez and Poulsen, 2013). Based on the geomorphology of the paleo-fjord network and its glaciogenic sediment infill, we infer the pace of ice-margin fluctuations and the paleo-ice thickness that once occupied these fjords. Furthermore, we estimate glacio-isostatic adjustment following deglaciation. This work highlights the dynamic nature of the LPIA in southwestern Gondwana and the potential for an under-appreciated deep-time carbon and sediment sink that, if valid, would have had direct climate implications through the impact of Carboniferous–Permian atmospheric CO2.The Kaokoland region of northwestern Namibia is characterized by a prominent bimodal topography where staircase-like plateaus (at ∼500 m, ∼1000 m, and ∼1500 m) separated by NNW-SSE–oriented escarpments are deeply dissected by a network of valleys in which modern rivers flow (Fig. 1A). These valleys are 1–5 km in width and 80–130 km in length and have steep, subvertical flanks defining U-shaped cross-profiles (Figs. 1B and Fig. 1C) whose depths range between 400 and 1200 m; interfluves occur up to 1.7 km above the thalweg of the Kunene Valley (Fig. 1A). These valleys are carved within hard Archean to Proterozoic lithologies of the Pan-African and Congo craton basement (Goscombe and Gray, 2008).The valley floors display abundant hard-bed glacial erosion features such as striae, scratches, grooves, and crescentic gouges (Fig. 2A) superimposed on whalebacks and roches moutonnées (see also Martin, 1953, 1981; Martin and Schalk, 1959). Scratches and striations are also developed on subvertical walls that flank the U-shaped valleys as well as on the subvertical westernmost (Purros; Fig. 1) escarpment, indicating westward and northward paleo-ice flows, respectively (Fig. 1A). These valleys ubiquitously preserve remnants of partly eroded glaciogenic sediments of the Dwyka Group, forming the base of the Karoo Supergroup (Figs. 1B, 1C, and 2), whose age in the (restored) neighboring Paraná Basin of Brazil and Aranos and Karasburg Basins of Namibia is bracketed between ca. 299 Ma and ca. 296 Ma (Griffis et al., 2021). These glaciogenic sediments consist here of boulder beds (Fig. 2B) encompassing numerous exotic, faceted, and/or striated clasts and discontinuous ridges, patches, and lenses of poorly sorted conglomerates commonly affected by syn-sedimentary ductile deformation (Figs. 1D and Fig. 2C), interpreted as ice-contact morainal banks or ridges that experienced glaciotectonic folding (Dowdeswell et al., 2015). Importantly, glaciogenic sediments encompassing numerous and large (1–3 m) glacial erratics are plastered to the sides of these U-shaped valleys and along the escarpment, in association with scratched and striated valley walls (Figs. 2D and Fig. 2E). Glaciogenic deposits occur consistently at a height of 100 m above the valley bottom, which are interpreted as marginal moraines.Sediments immediately covering the coarse glaciogenic deposits, infilling the valleys (Figs. 1C and 3) and abutting against the striated valley walls (Figs. 1B and Fig. 2D), are made up of a 5–20-m-thick shallowing-upward sequence of medium-grained sandstones with rhythmic climbing ripples and sand-mud couplets (Fig. 2F) interstratified with diamictite layers characterized by scattered outsized clasts and highlighted by impact structures reflecting ice-rafted debris (Fig. 2G). We interpret this succession to represent ice-proximal subaqueous fan deposits (Dowdeswell et al., 2015), implying a glaciomarine depositional environment. Facies characteristic of intertidal processes (truncated, leveled, and breached wave ripples; Fig. 2H) were deposited in a postglacial context, 20 m above the striated floors, well below valley interfluves (Figs. 1D and 3). The fjords were subsequently drowned and accumulated non-glaciogenic sedimentary units of the Permian–Cretaceous Karoo Supergroup, ultimately culminating with the Etendeka basalt flows at 130 Ma, whose remnants occur in valley axes topping both the sedimentary succession and valley interfluves (Fig. 1).The presented glaciogenic sediments and geomorphic features preserved within and along the sides of these U-shaped valleys and escarpments indicate that they were occupied by ice masses during the LPIA. Ice retreat was accompanied by postglacial marine incursion that therefore turned the valleys into fjords; the observed valley network therefore corresponds to the original fjord network, here mapped for the first time in Figure 1A.The glacial (fjord) geomorphology and its associated sedimentary infill record a snapshot of the dynamics of the LPIA ice masses (Fig. 4). The deep (as deep as 1.7 km) fjord incisions and the presence of numerous and large glacial erratics (e.g., Fig. 2E) transported by ice across major drainage divides imply the presence of an ice sheet largely overflowing the valleys during an early glaciation acme phase (Fig. 4; Martin and Schalk, 1959; Staiger et al., 2005; Kessler et al., 2008; Steer et al., 2012 ; Livingstone et al., 2017), with ice of at least 1.7 km thick flowing likely westward toward southeastern Brazil. During the subsequent waning phase, an estimated ice thickness of 100 m existed within the valley, as extrapolated directly from the elevation difference between the valley bottom and the highest observed marginal moraine sediments plastered on the valley flanks (Figs. 1 and 3). As indicated by such an ice thickness, topographically constrained, westward-flowing outlet glaciers occupied the valleys, and some (such as the glaciers that occupied the Gomatum and Hoarusib valleys; Fig. 1) bifurcated or fed into a bigger, northward-flowing ice stream (Fig. 4). These outlet glaciers are therefore regarded as having drained a residual ice sheet located to the east of the study area, possibly over the Otavi Range (Fig. 1A) or the Owambo Basin further east (Miller, 1997). Our findings therefore demonstrate that both a large ice sheet and highland glaciers existed over northwestern Namibia during particular intervals of the icehouse. As is the case for the Quaternary period, however, for which only the last cycle is preserved in the sedimentary and geomorphic record, this acme-waning succession may be representative of only an ultimate ice growth and decay cycle, and perhaps the most important one, of a period that encompassed several cycles that were erased by subsequent ice advance.Throughout this acme-waning cycle, during ongoing deglaciation and ice-margin retreat, the postglacial sea invaded the valleys and turned them into fjords. Ice-contact fans and deformed morainal banks indicate that sedimentation took place at the front of retreating glaciers during periods of episodic ice-margin stillstands or minor readvance (Fig. 4). Throughout this deglacial cycle, ice-contact and glaciomarine sedimentation was strictly confined within the topographic depression (the fjords) in a setting likely devoid of tectonic subsidence.Considering such a succession of deglacial events, the sedimentary succession observed infilling the fjords is therefore thought to archive the glacio-isostatic adjustment (Boulton, 1990). Given that glacio-isostatic adjustment operates over a short (104 yr) time scale, i.e., well shorter than the temporal resolution available for such a deep-time record, the unravelling of this process is made by analogy between the fjordal deglacial sedimentary succession showcased here and a Quaternary deglacial succession (Dietrich et al., 2018). Thus, we posit that the basal, 20–40-m-thick shallowing-upward parasequence (glaciomarine capped by intertidal facies) is interpreted as a response to falling relative sea level resulting from the glacio-isostatic adjustment. The subsequent drowning (offshore deposits upon intertidal deposits) therefore likely corresponds to an eustatic rise during which non-glaciogenic sedimentation occurred (Fig. 3). The glacial dynamic that we envisage is largely comparable to the evolution of post–Last Glacial Maximum ice masses over Canada, Norway, and Greenland, whose post-acme recession saw the confinement of outlet glaciers into fjords, which after glacial demise were invaded by postglacial seas (e.g., Syvitski et al., 1987; Dietrich et al., 2018).The preservation of glaciogenic deposits and erosion features indicates that the U-shaped valley network forms an intact relict geomorphic landscape inherited from the LPIA and complements temporally contemporaneous, though smaller, glacial landscapes and paleo-fjords preserved and exhumed (or still sealed) across southwestern Gondwana (e.g., Visser, 1987; Assine et al., 2018; Le Heron et al., 2019; Fallgatter and Paim, 2019). The presence of incised valleys furthermore implies that the plateaus and intervening escarpments were in place when the glacial topography was carved (Fig. 1D), i.e., before the Atlantic (Cretaceous) rifting. Thermochronological studies (Krob et al., 2020) indicate that little denudation occurred after the LPIA prior to the deposition of the Cretaceous Etendeka basalts, whose remnants are preserved on valley interfluves (Fig. 1C). Therefore, modern valley depths and the whole of the Kaokoland landscape, although uplifted (Baby et al., 2020), are virtually unchanged since LPIA times (Martin, 1953). The observed network of valleys, troughs, and escarpments that currently characterizes the Kaokoland therefore corresponds to an extensive, ∼50,000 km2 preserved glacial landscape (Fig. 1A). Our showcased example is unique because it represents the sole example of a pristine fjord network and glacial landscape yet described for a pre-Cenozoic glacial epoch. Furthermore, our work highlights the compatibility of both large ice sheets and highland glaciation across the LPIA in a single location (cf. Isbell et al., 2012).In the absence of recent (middle to late Paleozoic) major tectonic events antecedent to the LPIA in this region, the preexistence of plateaus and escarpments exploited by glacial erosion that carved fjords is interpreted as follows. The NNW-SSE–oriented escarpments correspond to basement sutures delineating the Congo craton to the west and segments of the Kaoko (Pan-African) orogen to the east (Goscombe and Gray, 2008). Because these plateaus and escarpments had already formed prior to the LPIA, we suggest that this existing topography resulted from substantial rejuvenation of the Kaoko orogenic structures after 200 Ma during post-orogenesis exhumation and peneplanation (Krob et al., 2020). A differential response of the Congo craton and the Kaoko orogen to vertical tectonic forces that promoted the initiation of Karoo-aged basins over southwestern Gondwana (Pysklywec and Quintas, 1999) is tentatively invoked to explain this topographic rejuvenation. Alternatively, or complementarily, glacial erosion itself through isostatic uplift may have generated the mountainous relief required to create fjords (Medvedev et al., 2008). After the LPIA and until 130 Ma, this paleo-fjord network was progressively buried by the Karoo sediments and Etendeka volcanics, and subsequently exhumed until the present (Krob et al., 2020; Margirier et al., 2019; Baby et al., 2020). Thus, preservation of a pristine paleo-fjord network, in spite of 130 m.y. of uplift and exhumation, is remarkable. Determining the reasons for this exceptional preservation will be a driver of future research.The Namibian paleo-fjords have major implications for understanding the turnover from the late Paleozoic icehouse to a permanent greenhouse state. Delineating the extent and dynamics of ice masses in northwestern Namibia provides more realistic boundary conditions for the scale of glaciation in this region of Gondwana. In particular, the reconstructed paleo-landscape that requires an ice sheet during the acme followed by upland glaciation through the demise of the LPIA could well be explained by the insertion of fjords that promoted dramatic ice-mass loss through drainage and ablation (Bennett, 2003; Briner et al., 2009), in turn triggering abrupt climate change and enhanced ice-sheet sensitivity to climate change, ultimately leading to ice shrinkage (Kessler et al., 2008). Importantly, the Namibian paleo-fjord network together with its South American (Tedesco et al., 2016, and references therein) and South African (Visser, 1987) counterparts could have facilitated the deposition and long-term sequestration of organic carbon analogous to Quaternary fjords (Smith et al., 2015). If this was the case, then burial of large amounts of glacially derived organic material may have contributed to a 10 m.y. nadir in atmospheric CO2 in the earliest Permian that defines a paradox, given the loss of major carbon sinks prior to the close of the Carboniferous (Richey et al., 2020). The potential for paleo-fjords of southwestern Gondwana to have played an important role in atmospheric pCO2 and climate regulation is thus a worthwhile area of further study and carbon-cycle modeling.P. Dietrich and D. Le Heron acknowledge funding from the South Africa–Austria joint project of the National Research Foundation (NRF) of South Africa and the Österreichischer Austauschdienst (OEAD project ZA 08/2019). I. Montañez and N. Griffis acknowledge funding from the U.S. National Science Foundation (grant EAR-1729882). Julia Tedesco and Michael Blum are thanked for their thorough and constructive reviews that greatly led to the improvement of the paper.

中文翻译:

纳米比亚的峡湾网络:古生代晚期冰川作用的快照

峡湾是冰川雕刻的河口,通过排水和消融冰对冰盖的稳定性产生深远的影响。尽管在现代高纬度大陆架上有着丰富的峡湾网络形态,但在地球的前新生代冰川时期从未发现过峡湾网络形态,这阻碍了我们限制古代冰盖动力学的能力。我们展示了纳米比亚西北部的 U 形山谷在晚古生代冰河时代(LPIA,约 300 Ma),地球的倒数第二个冰库,代表了完整的峡湾网络形态。这种保存完好的冰川形态及其沉积填充物允许重建古冰厚度、冰川动力学以及由此产生的冰川均衡调整。该地区的冰川作用最初以顶点阶段为特征,该阶段覆盖了广泛的冰盖(1.7 公里厚),随后是一个衰退阶段,其特征是 100 米厚、受地形限制的出口冰川缩小,导致冰川消亡。我们的研究结果表明,在 LPIA 期间,纳米比亚西北部的不同时间都存在大冰盖和高原冰川。峡湾可能在冰川动力学和气候调节中发挥了关键作用,是有机碳封存的热点。除了现在的干旱气候,纳米比亚西北部的地貌自 LPIA 以来几乎没有变化,这使人们可以对这座冰库进行独特的了解。峡湾是长而深且狭窄的冰川雕刻河口,被出口冰川占据。它们在冰盖稳定性(即排水和消融)方面发挥着重要作用,对冰库期间的气候变化很敏感,并充当重要的沉积物汇(Syvitski 等,1987;Bennett,2003;Kessler 等,2008;Briner 等,2009;Moon 等,2018)。尽管仅占地球现代海洋的 0.1%,但峡湾占地球有机碳埋藏的 10% 以上,因此可能对全球碳循环产生重大影响(Smith 等,2015)。此外,峡湾形态和沉积填充物在评估冰盖动态和气候变化方面发挥着重要作用(Eilertsen 等,2011;Steer 等,2012;Normandeau 等,2019;Bianchi 等,2020)。尽管现在高纬度大陆架上的峡湾丰度很高,但主要是从地层关系和地质图推断出来自前新生代冰川时期的峡湾(例如,Kneller 等人,2004 年;Tedesco 等人,2016 年,以及其中的参考资料) ), 而古峡湾网络的存在尚未严格确定。古代峡湾形态的稀缺似乎反映了这些大规模地貌特征的内在瞬态特征(Bianchi 等,2020),使它们在地质时间尺度上容易受到侵蚀。反过来,我们全面了解深时冰盖的大规模动态并评估其对气候变化的敏感性的能力受到阻碍。地球的倒数第二个长寿冰库,晚古生代冰河时代(LPIA,360-260 Ma;Montañez 和 Poulsen,2013 年)。基于古峡湾网络及其冰川沉积物填充物的地貌,我们推断冰缘波动的速度和曾经占据这些峡湾的古冰层厚度。此外,我们估计冰川消融后的冰川均衡调整。这项工作突出了冈瓦纳西南部 LPIA 的动态性质,以及未被充分认识的深时碳和沉积物汇的潜力,如果有效,将通过石炭纪 - 二叠纪大气 CO2 的影响对气候产生直接影响。 Kaokoland纳米比亚西北部地区以突出的双峰地形为特征,其中阶梯状高原(约 500 米、约 1000 米和约 1500 米)被 NNW-SSE 取向的悬崖隔开,被一个山谷网络深深分割,其中现代河流流动(图1A)。这些山谷宽 1-5 公里,长 80-130 公里,有陡峭、定义 U 形横截面的近垂直侧面(图 1B 和图 1C),其深度范围在 400 和 1200 m 之间;间流发生在库内内山谷 thalweg 上方 1.7 公里处(图 1A)。这些山谷雕刻在泛非和刚果克拉通基底的太古宙至元古代坚硬岩性中(Goscombe 和 Gray,2008 年)。谷底显示出丰富的硬层冰川侵蚀特征,如条纹、划痕、凹槽和新月形凿孔(图 2A) 叠加在鲸背和 roches moutonnées 上(另见 Martin,1953,1981;Martin 和 Schalk,1959)。划痕和条纹也在 U 形山谷两侧的近垂直壁以及近垂直最西端(Purros;图 1)的悬崖上发育,分别表明向西和向北的古冰流(图 1A)。这些山谷无处不在地保存着德威卡群部分侵蚀冰川沉积物的残余物,形成了卡鲁超群的底部(图 1B、1C 和 2),其年龄在(恢复的)邻近的巴西巴拉那盆地以及阿拉诺斯和卡拉斯堡盆地纳米比亚的括号括在大约之间。299 马和约。296 Ma(格里菲斯等人,2021 年)。这些冰川沉积物由巨石床组成(图 2B),其中包括许多奇异的、多面的和/或条纹状的碎屑和不连续的脊、斑块和分选不良的砾岩透镜,这些砾岩通常受同沉积延展性变形的影响(图 1D 和图 1D) . 2C),解释为经历冰川构造折叠的冰接触碛堤或山脊(Dowdeswell 等,2015)。重要的,包含大量大(1-3 m)冰川不规则物的成冰沉积物被涂抹在这些 U 形山谷的侧面和悬崖上,与划痕和条纹的谷壁有关(图 2D 和图 2E)。成冰沉积物始终出现在谷底以上 100 m 的高度,被解释为边缘冰碛。沉积物立即覆盖粗大的成冰沉积物,填充山谷(图 1C 和 3)并紧靠条纹谷壁(图 1C 和图 3)。图 1B 和图 2D),由 5-20 米厚的浅层中粒砂岩组成,具有有节奏的攀爬波纹和砂泥对联(图 2F),其间夹杂以分散的超大尺寸为特征的混叠岩层。碎屑并通过反映冰筏碎片的撞击结构突出显示(图 2G)。我们将这种演替解释为代表近冰层的水下扇形沉积物(Dowdeswell 等,2015),暗示着冰川沉积环境。潮间带过程的特征相(截断的、水平的和破裂的波浪纹;图 2H)沉积在冰河期背景下,条纹底面上方 20 m,远低于谷间河道(图 1D 和 3)。峡湾随后被淹没并积累了二叠纪-白垩纪卡鲁超群的非冰川成因沉积单元,最终在 130 Ma 时以 Etendeka 玄武岩流达到顶峰,其残余物出现在沉积序列和河谷交汇处顶部的山谷轴中(图 1) . 在这些 U 形山谷和悬崖的内部和两侧保存的冰川沉积物和地貌特征表明它们在 LPIA 期间被冰块占据。冰川退缩伴随着冰后海洋入侵,因此将山谷变成了峡湾;因此,观察到的山谷网络对应于原始峡湾网络,这里首次绘制在图 1A 中。冰川(峡湾)地貌及其相关的沉积填充记录了 LPIA 冰团动态的快照(图 4)。深(深达 1.7 公里)的峡湾切口和大量和大型冰川不规则的存在(例如,图 2E)由冰穿过主要的排水沟运输,这意味着在早期冰川作用达到顶峰期间存在大量溢出山谷的冰盖相(图 4;马丁和沙尔克,1959 年;斯泰格等人,2005 年;凯斯勒等人,2008 年;斯蒂尔等人,2012 年;Livingstone 等人,2017 年),至少 1.7 公里厚的冰可能向西流向巴西东南部。在随后的衰退阶段,根据谷底和观察到的覆盖在谷侧的最高边缘冰碛沉积物之间的海拔差异直接推断,谷内存在估计为 100 米的冰厚(图 1 和图 3)。正如这样的冰厚所表明的那样,受地形限制的向西流动的出口冰川占据了山谷,并且一些(例如占据 Gomatum 和 Hoarusib 山谷的冰川;图 1)分叉或流入更大的向北流动的冰流(图 4)。因此,这些出口冰川被认为已经排干了位于研究区东部的残余冰盖,可能在奥塔维山脉(图 1A)或更东的奥万博盆地(米勒,1997 年)。因此,我们的研究结果表明,在冰库的特定间隔期间,纳米比亚西北部存在大冰盖和高原冰川。然而,与第四纪的情况一样,在沉积和地貌记录中只保留了最后一个周期,这种逐渐减弱的序列可能只代表最终的冰生长和衰变周期,也许是最重要的一个, 一个包含几个周期的时期,这些周期被随后的冰层推进所抹去。在这个逐渐减弱的周期中,在持续的冰川消退和冰缘退缩期间,冰河期后的海水侵入了山谷,将它们变成了峡湾。冰接触扇和变形的冰碛堤表明,在偶发性冰缘静止或小幅上升期间,在退缩的冰川前部发生了沉积(图 4)。在整个冰消期循环中,冰接触和冰川沉积被严格限制在可能没有构造沉降的地形凹陷(峡湾)内。 考虑到这种冰消期事件的连续性,因此认为观察到的填充峡湾的沉积连续性存档冰川均衡调整(Boulton,1990)。鉴于冰川均衡调整在很短(104 年)的时间尺度上运行,即远短于可用于此类深度记录的时间分辨率,这个过程的解体是通过这里展示的峡湾冰消沉积序列与第四纪冰消沉积序列之间的类比来完成的(Dietrich et al., 2018)。因此,我们假设基础的、20-40 米厚的浅上层副层序(冰川被潮间相覆盖)被解释为对冰川均衡调整导致的相对海平面下降的响应。因此,随后的淹没(潮间带沉积物上的近海沉积物)可能对应于非冰川沉积发生期间的海平面上升(图 3)。我们设想的冰川动态在很大程度上与加拿大、挪威和格陵兰岛末次盛冰期后冰团的演变相当,后者的顶峰衰退后出口冰川被限制在峡湾中,冰川消亡后被冰后海侵入(eg, Syvitski et al., 1987; Dietrich et al., 2018)。 冰川沉积和侵蚀特征的保存表明U型河谷网络形成了一个完整的遗存地貌景观。来自 LPIA 并补充了在冈瓦纳西南部保存和挖掘(或仍然密封)的时间同期但较小的冰川景观和古峡湾(例如,Visser,1987 年;Assine 等人,2018 年;Le Heron 等人,2019 年; Fallgatter 和 Paim,2019 年)。此外,切割山谷的存在意味着,当冰川地形被雕刻时(图 1D),即在大西洋(白垩纪)裂谷之前,高原和中间的悬崖就位。热年代学研究(Krob 等人,2020) 表明在白垩纪 Etendeka 玄武岩沉积之前 LPIA 之后几乎没有发生剥蚀,其残余物保存在山谷间河道上(图 1C)。因此,现代山谷的深度和整个 Kaokoland 地貌虽然抬升(Baby 等,2020),但自 LPIA 时代以来几乎没有变化(Martin,1953)。因此,目前表征 Kaokoland 的山谷、海槽和悬崖的观测网络对应于一个广泛的、约 50,000 平方公里的保存冰川景观(图 1A)。我们展示的例子是独一无二的,因为它代表了原始峡湾网络和冰川景观的唯一例子,但仍被描述为前新生代冰川时期。此外,我们的工作强调了大冰盖和高地冰川在同一地点跨越 LPIA 的兼容性(参见 LPIA)。Isbell 等人,2012 年。在该地区缺乏 LPIA 之前的近期(中古生代)主要构造事件的情况下,对雕刻峡湾的冰川侵蚀所开发的高原和悬崖的预先存在解释如下。NNW-SSE 方向的悬崖对应于描绘西部刚果克拉通和东部 Kaoko(泛非)造山带部分的基底缝合线(Goscombe 和 Gray,2008)。因为这些高原和悬崖在 LPIA 之前就已经形成,我们认为这种现有的地形是由于在 200 Ma 之后在造山后剥脱和剥蚀过程中 Kaoko 造山结构的大量恢复(Krob et al., 2020)。刚果克拉通和 Kaoko 造山带对垂直构造力的不同响应促进了冈瓦纳西南部卡鲁时代盆地的形成(Pysklywec 和 Quintas,1999 年)被暂时援引来解释这种地形复兴。或者,作为补充,冰川侵蚀本身通过均衡隆起可能产生了形成峡湾所需的山地起伏(Medvedev 等人,2008 年)。在 LPIA 之后直到 130 Ma,这个古峡湾网络逐渐被 Karoo 沉积物和 Etendeka 火山岩掩埋,随后被挖掘直到现在(Krob 等,2020;Margirier 等,2019;Baby 等, 2020)。因此,尽管经历了 130 米的隆起和挖掘,原始古峡湾网络的保存仍是显着的。确定这种特殊保存的原因将是未来研究的驱动力。纳米比亚古峡湾对于理解从晚古生代冰库到永久温室状态的周转具有重要意义。描绘纳米比亚西北部冰团的范围和动态为冈瓦纳大陆地区的冰川规模提供了更现实的边界条件。特别是,重建的古景观需要冰盖在顶点期间形成,然后通过 LPIA 的消亡进行高地冰川作用,这可以很好地解释为插入峡湾,通过排水和消融促进冰量急剧减少(Bennett,2003 年) ; Briner et al., 2009),进而引发突然的气候变化并增强冰盖对气候变化的敏感性,最终导致冰收缩(Kessler 等人,2008 年)。重要的是,纳米比亚古峡湾网络及其南美洲(Tedesco 等人,2016 年,以及其中的参考文献)和南非(Visser,1987 年)对应物可能促进了类似于第四纪的有机碳的沉积和长期封存。峡湾(史密斯等人,2015 年)。如果是这种情况,那么考虑到石炭纪末期主要碳汇的损失,埋藏大量冰川衍生的有机物质可能导致最早的二叠纪大气 CO2 的最低点为 10 my 最低点,这定义了一个悖论(Richey 等人,2020 年)。冈瓦纳西南部古峡湾在大气 pCO2 和气候调节中发挥重要作用的潜力因此是一个值得进一步研究和碳循环建模的领域。Dietrich 和 D. Le Heron 承认来自南非国家研究基金会 (NRF) 和 Österreichischer Austauschdienst(OEAD 项目 ZA 08/2019)的南非-奥地利联合项目的资助。I. Montañez 和 N. Griffis 承认来自美国国家科学基金会的资助(授予 EAR-1729882)。感谢 Julia Tedesco 和 Michael Blum 对论文的全面和建设性的评论,这些评论极大地促进了论文的改进。Le Heron 承认来自南非国家研究基金会 (NRF) 和 Österreichischer Austauschdienst(OEAD 项目 ZA 08/2019)的南非-奥地利联合项目的资助。I. Montañez 和 N. Griffis 承认来自美国国家科学基金会的资助(授予 EAR-1729882)。感谢 Julia Tedesco 和 Michael Blum 对论文的全面和建设性的评论,这些评论极大地促进了论文的改进。Le Heron 承认来自南非国家研究基金会 (NRF) 和 Österreichischer Austauschdienst(OEAD 项目 ZA 08/2019)的南非-奥地利联合项目的资助。I. Montañez 和 N. Griffis 承认来自美国国家科学基金会的资助(授予 EAR-1729882)。感谢 Julia Tedesco 和 Michael Blum 对论文的全面和建设性的评论,这些评论极大地促进了论文的改进。
更新日期:2021-11-23
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