Human impacts on large benthic foraminifers near a densely populated area of Majuro Atoll, Marshall Islands

Human impacts on sand-producing, large benthic foraminifers were investigated on ocean reef flats at the northeast Majuro Atoll, Marshall Islands, along a human population gradient. The densities of dominant foraminifers Calcarina and Amphistegina declined with distance from densely populated islands. Macrophyte composition on ocean reef flats differed between locations near sparsely or densely populated islands. Nutrient concentrations in reef-flat seawater and groundwater were high near or on densely populated islands. δ¹⁵N values in macroalgal tissues indicated that macroalgae in nearshore lagoons assimilate wastewater-derived nitrogen, whereas those on nearshore ocean reef flats assimilate nitrogen from other sources. These results suggest that increases in the human population result in high nutrient loading in groundwater and possibly into nearshore waters. High nutrient inputs into ambient seawater may have both direct and indirect negative effects on sand-producing foraminifers through habitat changes and/or the collapse of algal symbiosis.     

High‐frequency subbottom reflection types and lithologic and physical properties of sediments

Abstract

Eight types of reflections are interpreted from 3,800 km of 3.5 kHz profiles taken over a 25,000 km² area of the upper continental slope and shelf in the northeastern Gulf of Mexico off Panama City, Florida. The corresponding sediments in five of the reflection types were sampled in 77 piston cores from which data were obtained on in situ acoustic velocities (V), bulk densities (gr), sediment texture (mean grain size = Mz), CaCO₃ content (C), sedimentary structures, and gross sediment composition. A distinct bottom echo with numerous subbottom reflectors (Type I) is observed in deeper areas where terrigenous clay or lutite (Mgi = 9.9 to, gr = 1.4 g/cc, porosity (P) = 74 percent, C = 28 percent, and V (upper 2 m) = 1,435 m/s) predominates. Type I reflection grades upslope into Type IV, which shows a distinct bottom echo with fewer subbottom reflectors, and the corresponding sediment is a foraminiferal silty clay (mz = 9.4 to, gr = 1.43 g/cc, P = 73 percent, V = 1,447 m/s, and C = 37 percent). The uppermost slope gives indistinct, semiprolonged bottom echoes with faint subbottoms (Type VI) where calcareous silt (Mz = 6.6 to, gr = 1.57 g/cc, P = 65 percent, C = 70 percent, and V = 1,482 m/s) is the main sediment type. The shelf sediments (gr = 1.66 g/cc, P = 58 percent, V = sl1,530 m/ s), varying from coarse silt (Mz = 5.3 to) to very coarse sand (Mz = ‐0.3 to) and 25 to 100 percent carbonate, show indistinct, semiprolonged bottom echoes with intermittent or mushy subbottoms (Type VII). Prolonged echoes with no subbottoms (Type VIII) are observed in areas where algal sands of variable grain size (Mz ‐ ‐0.9 to 2.7 to, gr = 1.66 g/cc, P = 59 percent, V = 1,530 to 1,690 m/s) occur.

The major trends in reflection types (loss in depth of penetration, loss in number of reflectors, and prolongation of initial bottom reflections) follow gradients of sedimentary and physical properties of the sediments, which are increases in mean grain size, bulk density, in situ acoustic velocity, CaCO₃ content, and decrease in porosity. Increases in the reflection coefficient and attenuation of the sound energy in the shelf sediments are probably important factors in the observed decrease in the depth of penetration of the sound energy in the shelf sediments.     

地质记录中的海水白云岩化作用

地质记录中的海水白云岩化作用是广泛存在的。它不仅发育于时代较新的始新世地层内(如太平洋中的 Enewetak 珊瑚岛地区),而且也发育于时代较老的震旦纪灯影期地层内(如云南东北部地区)。在以前的白云岩成因研究过程中,海水白云岩化作用的普遍性和重要性被大大地低估了。自从 Saller(1984)首次报道了正常海水白云岩化的实例以来,海水白云岩化作用的研究已取得了长足的进展,如 Kasner(1984);Land(1985);Smart(1988);Tucker(1990);朱同兴、罗安屏(1993)等。近年来,作者通过对云南东北部地区震旦系灯影组潮缘沉积物的岩石学、沉积相、古地理格局和沉积地球化学标志等综合研究,认为该套巨厚的白云岩层(400—1000m)的成因不是原生的,而是成岩交代形成的,其主要的形成机制之一就是成岩早期的海水白云岩化作用。海水白云岩化作用的发育主要受海平面变化、沉积作用类型以及古气候等条件的限制。造成滇东北地区灯影期海水白云岩化作用的主要机制可能是位于其西北部的青藏大洋内的冷洋流向东部沉积物内泵吸,和位于其东南缘的扬子边缘海的海水向西部沉积物内循环对流(柯特对流)。上述的泵吸作用与对流作用的相互叠加可使巨大体积的海水通过扬子海域,尤其是扬子台地西南边缘(滇东北地区)沉积物,从而使其发生巨大规模的海水白云岩化作用。由海水白云岩化作用形成的白云岩的结构有序度中等,I=0.68—0.82,其碳、氧同位素组成均显示海水碳酸盐来源,δ~(13)=+1.8—+2.6‰(PDB),δ~(18)O=+2.1—+2.7‰(PDB)(冷洋流来源,Enewetak Atoll,始新统)或-5.1—-6.5‰(受地热和大气淡水双重影响而偏负值,滇东北地区;灯影组)。灯影期海水白云岩化作用模式合理地解释了区域巨厚沉积物的块状白云岩化成因、充足的镁离子来源以及多余的钙离子的搬运等白云岩成因研究过程中的关键性问题。     

The effect of a natural water-movement related disturbance on the structure of meiofauna and macrofauna communities in the intertidal sand flat of Rocas Atoll (NE, Brazil)

Rocas, the only atoll in the South Atlantic, is located 266 km off the northeast Brazilian coast. Spatial patterns in community structure of meiofauna, particularly nematodes, and macrofauna were examined along a transect through the sediment path from windward to leeward of the Rocas Atoll sand flat. Differences in benthic community structure between four zones of the sand flat were found to be significant and related to the major local processes of carbonate-grain transport and sedimentation. Both meiobenthic and macrobenthic assemblages were significantly more diverse and abundant within the sediment inflow zone (the initial part of the detrital path of Rocas sand flat) than in the other zones, where a clear impoverishment of benthic invertebrates occurred. This first study of the benthos of an intertidal sand flat over a reef island in the Atlantic showed that the meiofauna is numerically dominated by the nematodes Metoncholainussp. 1 (Oncholaimidae) and Epsilonema sp. 1 (Epsilonematidade), whilst the macrofauna is largely dominated by oligochaetes and large Oncholaimidae nematodes. Analysis of the species composition, trophic structure and abundance of both the meiobenthos and the macrobenthos revealed an impoverished community subjected to an intense water-movement disturbance.     

南沙群岛永暑礁环礁潟湖的沉积速率与气候变化

永暑礁南永 3井岩心是我国首次完整地获得的环礁湖沉积高分辨率柱样 ,井深 5 90m ,全为松散的生物骨壳碎屑。通过高精度TIMS铀系测年计算了沉积速率。全井柱样全新世晚期 16 82aBP以来的平均沉积速率为 3 5mm/a,比同礁同期的礁坪沉积速率 1 2 1mm/a高 ,是高分辨率古环境研究的理想材料。以全湖沉积柱平均沉积速率为准 ,将全柱划分为 4个沉积速率高低交替阶段。这同祖国大陆近 170 0年来的气温波动、冷热分期的变化大体上一致 ,反映了南沙群岛地区对中国大陆乃至全球的气候变化的响应。     

Analysis on coral reefs mapping using SPOT5 data at the Dongsha Atoll

Coral reefs are an sensitive-to-environment complex marine ecosystem. The ecosystem of corals is rich in biodiversity. Remote sensing offers a powerful tool for categorizing coral reefs and is the most cost-effective approach for the large-scale reef survey. The Dongsha Atoll, more than 300 km2 with an average depth of 10 m, is located at the northern continental margin of the South China Sea. It has been abused by destructive fishing during recent decades. Three satellite imageries (Quickbird2, ETM and SPOT5) are used to evaluate the capabilities of SPOT5 imagery to provide data that are useful for categorizing the current distribution of coral reefs therein. During the data processing, unsupervised classification functions are adopted for ETM and SPOT5 data, while the supervised classification method is used for Quickbird2. The classes are (or not) merged into coral reef, and then will be operated by vectorization, simplification, and topological analysis. There are 1 331 coral reefs larger than 100 m2 with a detection limit of 3×3 pixels at the multi-band data of Quickbird2, which is taken as the comparison baseline. The results extracted from SPOT5 and ETM images are less in number and area than those from the Quickbird2 image, whereas the results from SPOT5 data are better than those of ETM data at the silty lagoon due to its higher resolution. SPOT5 XS band 2 fails to distinguish the deep substrate inside the atoll compared with ETM data because of its poor penetration capability. Only SPOT XS band 1 cannot be used to differentiate coral reef from sand bottom. Merging the SPOT5 multi-bands data with the spatial resolution of SPOT5 pan-data and referring to ETM imagery are expected to provide an optimal satellite-based approach for mapping of coral reefs.     

Influence of sedimentary setting on the use of magnetic susceptibility: examples from the Devonian of Belgium

Bulk magnetic susceptibility measurements on sedimentological samples from all geological periods have been used widely in the last two decades for correlations and as a proxy for sea‐level variations. This paper explores the link between magnetic susceptibility, depositional setting and environmental parameters. These environmental parameters include distal–proximal transects, microfacies successions and fourth‐order trends on different carbonate platform types (platform, ramp, carbonate mound or atoll) during different Devonian stages (Eifelian, Givetian and Frasnian). Average magnetic susceptibility values over a distal–proximal‐trending facies succession vary markedly with depositional setting. On carbonate platforms, average magnetic susceptibility generally increases towards the top of shallowing‐upward sequences. On a distal–proximal transect, average magnetic susceptibility is intermediate for the deepest facies, decreases for the reef belts and increases to a maximum in the back‐reef zone. In ramps and atolls, magnetic susceptibility trends clearly differ; average magnetic susceptibility generally decreases towards the top of shallowing‐upward sequences and is highest in the deepest facies. The strong relationship between magnetic susceptibility, facies and sequences implies a strong environmental influence. However, the different responses in the different platform types suggest that sea‐level changes leading to variation in detrital input is not the only parameter controlling average magnetic susceptibility values. Other primary or secondary processes also probably influenced magnetic mineral distribution. Primary processes such as carbonate production and water agitation during deposition are probably key factors. When carbonate production is high, the proportion of magnetic minerals is diluted and the magnetic susceptibility signal decreases. High water agitation during deposition will also selectively remove magnetic minerals and will lead to low average magnetic susceptibility values. These parameters explain the lowest values observed on the reef platform, inner ramp and atoll crown, which are all in areas characterized by higher carbonate production and greater water agitation during deposition. The lowest values observed in the lagoon inside the atoll crown can be related to detrital isolation by the atoll crown. However, other parameters such as biogenic magnetite production or diagenesis can also influence the magnetic signal. Diagenesis can change magnetism by creating or destroying magnetic minerals. However, the influence of diagenesis probably is linked strongly to the primary facies (permeability, amount of clay or organic matter) and probably enhanced the primary signal. The complexity of the signal gives rise to correlation problems between different depositional settings. Thus, while magnetic susceptibility has the potential to be an important correlation tool, the results of this investigation indicate that it cannot be used without consideration of sedimentary processes and depositional environments and without strong biostratigraphical control.     

南海宣德环礁东南海域地质灾害特征

宣德环礁作为我国南海诸岛礁开发利用程度最高的岛礁之一, 其周围海域地质灾害对岛礁基础工程建设及人类活动具有重大危害。为适应南海岛礁可持续发展的需要, 文章采用广州海洋地质调查局2018年采集的单道地震数据, 对宣德环礁东部斜坡带海域灾害地质因素进行综合解释和分析。结果表明, 研究区主要发育有海底滑坡、海底陡坎和断层3类地质灾害。海底滑坡是研究区主要灾害地质因素, 规模大小不一, 海底陡坎和断层为次要地质灾害。海底陡坎根据成因可分为礁体陡坎和滑坡陡坎, 礁体陡坎为礁盘形成过程中受海平面上升控制形成, 滑坡陡坎为海底滑坡导致。通过分析认为陡峭的地形和峡谷水道的侵蚀搬运作用是研究区地质灾害发育的主控因素。这些地质灾害对海洋工程建设及管线铺设均有不利的影响, 应尽量避开。     

南沙群岛永暑礁小潟湖岩心有孔虫动物群及其沉积环境

根据永暑礁小潟湖"南永3"井高分辨率的岩心材料,结合其高精度TIMS U系测年,系统研究了1 682 a B.P.以来环礁潟湖沉积柱样的有孔虫分类?类型与属种的组成,揭示了其垂直变化?有孔虫共48属62种,都是全新世-现代种,其中底栖有孔虫55种?其组成表征了印度洋-太平洋有孔虫区系和热带珊瑚礁相环境?即使由温凉水种浮游有孔虫偏多的时段所反映的表层水温也相对偏低,推测其变化幅度很小?底栖有孔虫茸刺距轮虫δ18O含量的变化曲线呈4峰4谷交替,反映了永暑礁海区近1 682 a以来的气候有4个凉期和4个暖期,同2 000a以来的全球气候变化周期一致?有孔虫种类的垂向变化也反映了海平面有些小变动?     

Blue-green algae and terrestrial limestone weathering on Aldabra Atoll: An S.E.M. and light microscope study

Blue-green algae have been observed to affect limestone weathering on Aldabra Atoll, Indian Ocean. Three different habitats can be identified on the rock surface, i.e. epilithic, chasmolithic, and endolithic. Algae in each habitat may affect weathering in various ways. Samples of blue-green algae and rock were taken from various terrestrial and coastal environments on Aldabra Atoll. Samples of limestone tablets and calcite crystals after one year in situ were also studied. Light and S.E.M. microscopy revealed that endolithic boreholes were present on many samples, especially those from frequently wetted sites, to a maximum depth of 800 μm. An ‘altered zone’ of micrite and algal filaments was also discovered in many samples. From morphological and petrographical evidence blue-green algal influences on weathering on Aldabra Atoll seem to be very complex and cannot easily be related to small scale landforms.