Reef growth patterns and the development of associated environments have been extensively studied from reef deposits from Holocene and previous interglacial highstands. However, reefs that grew during glacial lowstands are comparatively poorly understood. Here we show the formation of reef-flat and back-reef environments following rapid sea-level fall (15–20 mm yr–1 and 20–40 m in magnitude) during the Last Glacial Maximum (LGM) on the present shelf edge of the Great Barrier Reef. Sedimentological and foraminiferal analyses of unconsolidated reef sediments recovered in cores 111–140 m below sea level at Hydrographers Passage during Integrated Ocean Drilling Project (IODP) Expedition 325 reveal the occurrence of a benthic foraminiferal assemblage dominated by the genera Calcarina and Baculogypsina, which is common in modern reef-flat and back-reef environments in the Great Barrier Reef and elsewhere. This assemblage is associated with higher foraminiferal proportions in reef sediments and higher proportions of well-preserved Baculogypsina tests in the same intervals, which also characterize reef-flat environments. Radiocarbon (14C–accelerator mass spectrometry) ages of reef-flat dwelling foraminifers (n = 22), which indicate the time when these foraminifers were alive, are consistent with the timing of the two-step sea-level fall into the LGM as defined by the previously published well-dated coralgal record. This foraminiferal evidence suggests the development of geomorphically mature fringing reefs with shallow back-reef lagoons during the LGM. Our results also imply that back-reef sediment accumulation rates during the LGM lowstand were comparable to those during the Holocene highstand. INTRODUCTION Reef growth patterns during interglacial sealevel highstands have been well studied based on drilling of Holocene and last-interglacial reef deposits (Montaggioni, 2005; Woodroffe and Webster, 2014). Reef growth has been characterized by three main patterns: “keep up”, “catch up”, and “give up”, in addition to “prograded” and “backstepped” (Woodroffe and Webster, 2014; Camoin and Webster, 2015). In keep-up or catch-up mode, as reefs accrete vertically toward the sea surface, reef geomorphic zonation such as reef flat, back-reef lagoon, and fore-reef slope has been established. The accommodation space of a back-reef lagoon is gradually filled with transported reef-flat sediments as a result of wave action on exposed reef flats (e.g., Kennedy and Woodroffe, 2002; Smithers, 2011). Recently, reef growth patterns during the last-deglacial sea-level rise have been revealed (Camoin et al., 2012). However, reef growth patterns during glacial sea-level fall and glacial sea-level lowstands are yet to be fully understood (e.g., Esat and Yokoyama, 2006). During relative sea-level (RSL) fall due to eustasy, glacio-hydroisostasy, and/or tectonic uplift, an emergent give-up type reef typically occurs (Woodroffe and Webster, 2014; Camoin and Webster, 2015). Depending on the rate and magnitude of the RSL fall and available substrate, the reef system progrades (migrates seaward) and a new reef develops at a lower elevation (“turn-on”), but as a consequence, the entire preexisting reef is left stranded (“turn-off”). Typical examples are reef flats during the late Holocene sea-level fall, when highly productive coral reef flats changed to less-productive rubble and algal flats (e.g., Smithers, 2011; Harris et al., 2015). Rapid tectonic uplift can also cause similar back-reef turn-off and the turn-on of a new reef in a more seaward position (e.g., Webster et al., 1998). However, little is known about how reefs and associated environments respond to more rapid and larger-amplitude sea-level falls. Last Glacial Maximum (LGM) reefs have been observed at Mayotte (Indian Ocean), Vanuatu, Solomon Islands, Marquesas Islands (Pacific Ocean) (reviewed by Montaggioni, 2005), and Ryukyu Islands (Japan) (Sasaki et al., 2006) as thin veneers or coral communities 2–3 m thick, at depths of mostly 100–150 m below sea level (mbsl). More recently, sediment cores recovered from the shelf edge of the Great Barrier Reef during Integrated Ocean Drilling Project (IODP) Expedition 325 revealed that drowned Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/4857209/g46792.pdf by guest on 31 October 2019 2 www.gsapubs.org | Volume XX | Number XX | GEOLOGY | Geological Society of America shelf-edge reefs composed of in situ coralgal frameworks and associated microbialites developed during the LGM and the subsequent last deglaciation (Webster et al., 2018; Braga et al., 2019; Humblet et al., 2019). In addition, combined paleo–water depths and >580 radiometric ages of the coralgal assemblages revealed a two-step sea-level plunge, each of several tens of meters in magnitude and a change rate of 15–20 mm yr–1, into the LGM (Yokoyama et al., 2018). In response to these sea-level changes, five reef sequences have migrated vertically and laterally since 30 ka (Webster et al., 2018). Foraminiferal assemblages from unconsolidated reef sediments in the IODP Expedition 325 cores have also closely tracked associated changes in depositional environments. In particular, shallowing-upward sequences observed in the LGM deposits are likely related to the two-step falls in sea level (Yagioka et al., 2019). Here we show the formation of reef-flat and back-reef environments following rapid sea-level fall (at a rate of 15–20 mm yr–1 and magnitude of several tens of meters) during the LGM on what is now the Great Barrier Reef shelf edge, based on benthic foraminiferal assemblages, abundances, and taphonomic grades combined with radiocarbon dating of foraminiferal tests. We propose a model of reef growth, including reef-flat and back-reef generation, in response to rapid sea-level falls during the LGM. MATERIALS AND METHODS A total of 65 unconsolidated sediment samples were collected at depths from 111 to 140 mbsl from six cores retrieved from inner and outer submerged reef terraces along two transects (HYD-01C and HYD-02A; Fig. 1) from Hydrographers Passage (19.667°S, 150.417°E). The grain-size composition and benthic foraminiferal taxonomic composition data for 47 of the 65 samples have been reported by Yagioka et al. (2019); 18 additional samples were analyzed for those data types in this study. For each sample, sediments were washed, dry sieved, and weighed (weight percent) to estimate grain-size composition. Sediments of 2–0.5 mm size fraction were split with a splitter until a subsample contained >150 benthic foraminifer tests. All benthic foraminiferal specimens were picked from the subsample, identified to the lowest taxonomic level possible, and counted. More than 500 particles in another subsample were identified and counted to estimate bioclast compositions for each sample. Preservation states of >100 tests of Baculogypsina sphaerulata picked from an additional subsample were categorized in each sample, mainly based on abrasion states of spines (the number of spines remained), into grade S (all spines remaining), grade A (some spines remaining), grade B (the roots of spines remaining), and grade C (no spines or roots remaining). The proportion of stained tests of B. sphaerulata, which indicates reworking from exposed older deposits (Yordanova and Hohenegger, 2002), was also calculated among >100 Baculogypsina tests in each sample. 14C ages were measured on well-preserved tests (grades S, A, and B) of B. sphaerulata and Calcarina spp. from 22 selected samples (10–15 specimens for each sample). Accelerator mass spectrometry (AMS) analyses were performed using a single-stage AMS at the Atmosphere and Ocean Research Institute at the University of Tokyo (Japan). Conventional 14C ages were calibrated to calendar ages using OxCal v.4.3 (https://c14.arch.ox.ac.uk/oxcal.html) using the Marine13 curve (Reimer et al., 2013). Figure 1. (A) Map of study area showing location of two transects at Hydrographers Passage (HYD), Great Barrier Reef. Dotted line indicates a 100 m depth contour line, corresponding to the continental shelf break. (B,C) Topographic profiles of two transects, HYD-01C (B) and HYD-02A (C), showing location of drill sites (after Hinestrosa et al., 2016). The numbers shown next to drill traces indicate hole numbers of the Integrated Ocean Drilling Project Expedition 325. (D,E) Lithostratigraphy and stratigraphic distribution of foraminiferal assemblages (Webster et al., 2018; Yagioka et al., 2019), showing core depth intervals examined in this study (black rectangles) for HYD-01C (D) and HYD-02A (E). sed.—sediment; bst.—boundstone; mb.—microbialite; Unconsol.—unconsolidated. mbsl––meters below sea level; MIS—Marine Isotope Stage. (F) Four key taxa of benthic foraminifers, which characterize assemblages A (Baculogypsina sphaerulata and Calcarina capricornia) and B (Amphistegina lessonii and Amphistegina radiata). Scale bars in F are 0.1 mm. Shallowing upward A (back-reef to reef-margin zone) B (upper photic zone) C (intermediate to lower photic zone) D (modern shelf slope) Foraminiferal assemblage

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末次盛冰期 (30–17 ka) 期间海平面快速下降导致大堡礁的礁坪和后礁发育

已经从全新世和以前的间冰期高位的珊瑚礁沉积物中广泛研究了珊瑚礁生长模式和相关环境的发展。然而，对在冰川低位生长的珊瑚礁的了解相对较少。在这里，我们展示了在末次盛冰期 (LGM) 期间海平面快速下降（每年 15-20 毫米，震级 20-40 米）后形成的礁滩和后礁环境。大堡礁。在综合海洋钻探项目 (IODP) 远征 325 期间，对在海平面以下 111-140 m 的岩心中回收的松散珊瑚礁沉积物进行的沉积学和有孔虫分析表明，发生了以 Calcarina 和 Baculogypsina 属为主的底栖有孔虫组合，这在大堡礁和其他地方的现代礁滩和礁后环境中很常见。这种组合与珊瑚礁沉积物中较高的有孔虫比例和相同间隔内保存完好的 Baculogypsina 测试的比例较高有关，这也是礁滩环境的特征。礁滩栖居有孔虫（n = 22）的放射性碳（14C-加速器质谱）年龄，表明这些有孔虫存活的时间，与定义的两步海平面下降到 LGM 的时间一致由先前发表的过时的珊瑚虫记录。这一有孔虫证据表明，在 LGM 期间，地貌成熟的边缘礁与浅礁后礁泻湖的发展。我们的结果还意味着 LGM 低位期间的后礁沉积物积累速率与全新世高位期间的沉积速率相当。简介 基于全新世和末次间冰期珊瑚礁沉积物的钻探，已经对间冰期海平面高位期间的珊瑚礁生长模式进行了很好的研究（Montaggioni，2005 年；Woodroffe 和 Webster，2014 年）。珊瑚礁生长具有三种主要模式：“保持”、“赶上”和“放弃”，以及“退化”和“后退”（Woodroffe 和 Webster，2014 年；Camoin 和 Webster，2015 年）。在跟上或追赶模式下，随着礁体垂直向海面增生，形成礁坪、礁后泻湖、礁前坡等生物礁地貌分带。由于波浪作用在裸露的礁滩上，后礁泻湖的容纳空间逐渐充满了运输的礁滩沉积物（例如，Kennedy 和 Woodroffe，2002；Smithers，2011）。最近，已经揭示了末次冰消期海平面上升期间的珊瑚礁生长模式（Camoin 等，2012）。然而，冰川海平面下降和冰川海平面低位期间的珊瑚礁生长模式尚未完全了解（例如，Esat 和 Yokoyama，2006）。在相对海平面 (RSL) 由于浮华、冰川水均衡和/或构造抬升而下降期间，通常会出现紧急放弃型珊瑚礁（Woodroffe 和 Webster，2014 年；Camoin 和 Webster，2015 年）。根据 RSL 下降的速度和幅度以及可用底物，珊瑚礁系统前进（向海迁移），新的珊瑚礁在较低的海拔处发育（“开启”），但结果是，整个先前存在的珊瑚礁被搁浅（“关闭”）。典型的例子是全新世晚期海平面下降期间的礁滩，当时高产的珊瑚礁滩变成了低产的碎石和藻滩（例如，史密瑟斯，2011；哈里斯等，2015）。快速的构造抬升也会导致类似的后礁关闭和在更靠海的位置开启新礁（例如，Webster 等，1998）。然而，人们对珊瑚礁和相关环境如何对更快、幅度更大的海平面下降做出反应知之甚少。在马约特岛（印度洋）、瓦努阿图、所罗门群岛、马克萨斯群岛（太平洋）（由 Montaggioni 审查，2005 年）和琉球群岛（日本）（Sasaki 等人，2006 年）观察到末次盛冰期 (LGM) 珊瑚礁作为 2-3 m 厚的薄单板或珊瑚群落，在海平面以下 100-150 m (mbsl) 的深度。最近，在综合海洋钻探项目 (IODP) 远征 325 期间从大堡礁大陆架边缘回收的沉积物核心显示已淹死 下载自 https://pubs.geoscienceworld.org/gsa/geology/article-pdf/4857209/ g46792.pdf 来宾于 2019 年 10 月 31 日 2 www.gsapubs.org | 卷XX | 编号 XX | 地质 | 美国地质学会陆架边缘珊瑚礁由原位珊瑚框架和相关微生物岩组成，在 LGM 和随后的最后一次冰消期（Webster 等人，2018 年；Braga 等人，2019 年；Humblet 等人，2019 年）。此外，结合古水深和珊瑚群组合的 >580 辐射年龄揭示了两级海平面下降，每级下降数十米，变化率为 15-20 毫米 yr-1，进入 LGM（横山等人，2018 年）。为了响应这些海平面变化，自 30 ka 以来，五个珊瑚礁序列发生了垂直和横向迁移（Webster 等，2018）。来自 IODP Expedition 325 岩心中松散珊瑚礁沉积物的有孔虫组合也密切跟踪沉积环境的相关变化。特别是，在 LGM 沉积物中观察到的上浅层序很可能与海平面的两级下降有关（Yagioka 等，2019）。在这里，我们展示了在 LGM 期间海平面快速下降（每年下降 15-20 毫米，震级达数十米）后形成的礁滩和后礁环境。礁架边缘，基于底栖有孔虫组合、丰度和埋藏等级，结合有孔虫试验的放射性碳测年。我们提出了一个珊瑚礁生长模型，包括礁滩和后礁生成，以应对 LGM 期间海平面的快速下降。材料与方法 从 Hydrographers Passage (Hydrographers Passage) 的两个横断面 (HYD-01C 和 HYD-02A; 图 1) 内和外潜礁阶地取回的 6 个岩心，在 111 至 140 mbsl 的深度收集了总共 65 个未固结沉积物样品19.667°S，150.417°E）。Yagioka 等人报告了 65 个样品中 47 个的粒度组成和底栖有孔虫分类学组成数据。(2019); 本研究针对这些数据类型分析了另外 18 个样本。对于每个样品，将沉积物洗涤、干燥筛分并称重（重量百分比）以估计粒度组成。2-0.5 毫米大小的沉积物用分离器分离，直到含有 > 150 次底栖有孔虫测试。所有底栖有孔虫标本都是从子样本中挑选出来的，确定到可能的最低分类水平，并进行计数。鉴定并计数了另一个子样本中的 500 多个粒子，以估计每个样本的生物碎屑组成。从额外的子样本中提取的超过100个球状芽孢杆菌的保存状态在每个样本中进行分类，主要基于刺的磨损状态（剩余刺的数量），分为S级（所有刺剩余），A级（剩余部分刺） )、B 级（剩余刺的根）和 C 级（没有剩余刺或根）。B. sphaerulata 染色测试的比例，表明从暴露的旧沉积物返工（Yordanova 和 Hohenegger，2002），也在每个样本的 >100 Baculogypsina 测试中计算。14C 年龄是在保存完好的 B. sphaerulata 和 Calcarina spp 的测试（S、A 和 B 级）上测量的。来自 22 个选定的样本（每个样本 10-15 个样本）。加速器质谱 (AMS) 分析是在东京大学（日本）大气和海洋研究所使用单级 AMS 进行的。使用 Marine13 曲线（Reimer 等人，2013 年），使用 OxCal v.4.3 (https://c14.arch.ox.ac.uk/oxcal.html) 将传统的 14C 年龄校准为日历年龄。图 1. (A) 研究区域地图，显示大堡礁 Hydrographers Passage (HYD) 的两个横断面的位置。虚线表示 100 m 深的等高线，对应大陆架断裂。(B,C) HYD-01C (B) 和 HYD-02A (C) 两个横断面的地形剖面图，显示了钻探地点的位置（根据 Hinestrosa 等人，2016 年）。钻孔轨迹旁边显示的数字表示综合海洋钻探项目远征 325 的孔数。 (D,E) 有孔虫组合的岩石地层学和地层分布（Webster 等人，2018 年；Yagioka 等人，2019 年），显示了岩心深度本研究中检查的间隔（黑色矩形）用于 HYD-01C (D) 和 HYD-02A (E)。sed.——沉积物；bst.——边界石；mb.——微生物岩；Unconsol.——未合并的。mbsl——海平面以下米数；MIS——海洋同位素阶段。(F) 底栖有孔虫的四个关键分类群，其特征是组合 A（球茎杆藻和 Calcarina capricornia）和 B（Amphistegina Lessii 和 Amphistegina radiata）。F 中的比例尺为 0.1 毫米。向上浅 A（后礁到礁缘带） B（上光带） C（中到下光带） D（现代陆架坡） 有孔虫组合