Present-day sedimentation rates on the southern and southeastern Australian continental margins

Abstract

The Australian continental margin presents highly contrasted settings depending on (1) the presence or absence of a fluvial sediment supply, (2) the distance from the Australian mainland, and (3) the local hydrological setting. Despite the importance in surface area of the continental margin around the Australian mainland, so far only a few studies have dealt with sedimentation, most of them focusing on the northeastern Australian Shelf. This work presents the first large-scale investigation of modern sedimentation along the southern margin of Australia in the SE Indian Ocean, and the western margin of the Tasman Sea. Sedimentation intensity was assessed on a century timescale using a multi-tracer approach (²³⁴Th, ²¹⁰Pb, ²³²Th) on interface cores around the 1000 m water depth contour. ²³⁴Th (half-life: 24.1 days) in excess was detected in all surface samples, testifying to the occurrence of freshly deposited particles. Sedimentation and mass accumulation rates based on sedimentary ²¹⁰Pb excess profiles (CF:CS model) range between 0.027 and 0.280 cm y^–1 and between 14 and 222 mg cm^–2 y^–1, respectively. Whereas sedimentation rates are low and associated with carbonates on the western margin, sediments are more influenced by the detrital fraction and organic carbon on the eastern side of the continent. In comparison with northern continental margins (e.g. Timor Sea, Gulf of Papua), the southern Australian margin receives little sediment today, as it is rarely linked to a river system that would otherwise deliver large amounts of sediment, and also because of the presence of the extended shelf south of Australia.

澳大利亚大陆边缘呈现高度反差的环境，根据（1）河流沉积物来源的存在与否，（2）离澳洲大陆的距离，以及（3）本地水文环境。尽管澳洲大陆周围陆缘表面的重要性，迄今为止，仅少量研究涉及沉积作用，其中大部分集中在澳大利亚陆架东北部。本研究提出了对沿印度洋东南区的澳大利亚南缘和塔斯曼海西缘的现代沉积作用的第一次大规模调查。使用多示踪剂法（234Th, 210Pb, 232Th）在1000米水深海底的界面岩芯评估以世纪为尺度的沉积强度。在所有海底样品中都检测到过量的234Th（半衰期24.1天），证明新近沉积的颗粒的出现。基于沉积的210Pb，沉积作用和大量堆积率超过范围为0.027和0.280厘米/年和14至 222毫克​​厘米14之间和222毫克/​​厘米2/年的状态（CF：CS模式）CF：CS模式。在西缘，沉积速率低，与碳酸盐相关；然而，沉积物更受大陆东边的碎屑粒级和有机碳的影响。与北部大陆边缘（如帝汶海，巴布亚湾）比较，澳大利亚南部边缘现在接收很少沉积物，因为它很少与可能会提供大量沉积物的河流系统连接，也因为澳大利亚南陆架的延伸的存在。

KEY WORDS:

• Australian continental margin
• sedimentation rate
• mass accumulation rate
• thorium-234
• thorium-232
• lead-210
• particulate organic carbon
• carbonate
• grain size

ACKNOWLEDGEMENTS

The Australian National Oceans Office funded part of this project as well as ARC DP grant 110103926 awarded to PDD, SS and Stefan Schouten (Royal Netherlands Institute for Sea Research, NIOZ). Schmidt's salary was supported by the CNRS-INSU. Scientific members of both RV Southern Surveyor cruises helped in many ways with the use of the multicorer as well as the processing of the cores while at sea, and in particular Ashley Burkett, Chris Monday, Graham Nash, Marita Smith and Anthony E. Rathburn. We are also grateful to Don McKenzie of the Marine National Facility for all his support behind the scenes and Dr Tara Martin for much help with the swath mapping of potential core sites while at sea. We have benefited from the comments by H. Heijnis and an anonymous reviewer.

SUPPLEMENTAL DATA

Supplementary Table 1 Dry Bulk Density (g ^cm−3) (DBD) activities of ²¹⁰Pb, ²²⁶Ra, ²²⁸Th and ²³⁴Th_xs, mean grain size, percentage organic carbon (C_org) and percentage CaCO₃ content for the cores collected obtained along the western margin of the Tasman Sea during the SS2011-T1 cruise.

Supplementary Table 2 Dry Bulk Density (g ^cm−3) (DBD) activities of ²¹⁰Pb, ²²⁶Ra, ²²⁸Th and ²³⁴Th_xs, mean grain size, percentage organic carbon (C_org) and percentage CaCO₃ contents for the cores collected obtained along the southern margin of Australia during the SS2011-T4 cruise.