Holocene Reef Accretion: Southwest Molokai, Hawaii, U.S.A.
Holocene Reef Accretion: Southwest Molokai, Hawaii, U.S.A.
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全新世珊瑚礁增生：美国夏威夷摩洛凯岛西南部
DOI：
10.1306/073003740255
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发表时间：
2004
影响因子：
2
通讯作者：
C. Glenn
中科院分区：
文献类型：
--
作者：
M. Engels;C. Fletcher;M. Field;C. Storlazzi;E. Grossman;J. Rooney;C. L. Conger;C. Glenn
莫洛凯岛南部的Hale O Lono和Hikauhi两个礁系（相距仅10 km）在现代生态系统结构和全新世沉积历史上表现出强烈而根本的差异，这反映了波浪诱导的近床剪切应力对夏威夷礁发育的影响。这两个地点都受到来自南方、科纳和信风涌浪的类似影响。然而，Hale O Lono遗址暴露在北涌浪中，而Hikuahi遗址却没有。因此，Hale O Lono的礁体没有记录到晚全新世的净增生，而Hikauhi的礁体在晚全新世记录了持续而强劲的净增生。通过对Hale O Lono和Hikauhi地区24个岩心的分析和年代测定，揭示了反映古环境条件的5种主要岩相的存在。按沉积能量递减顺序依次为：(1)珊瑚-藻结合岩；(2)混合骨骼rudstone；(3)块状珊瑚框架岩；(4)松散浮石；(5)分枝珊瑚框架岩-挡板岩。在Hale O Lono， 10个岩心记录了一个后退的珊瑚礁，范围从8100 calyr BP（近海）到4800 calyr BP（近岸）。在15米深的地方，由于海浪的能量、被破坏的补充活动和物理磨损，一个衰竭的现代珊瑚群落在向海岸和向海方向减少。有证据表明，从全新世早期有利于增生的条件到全新世晚期不利于增生的条件发生了变化。从14个岩心重建了Hikauhi的礁体结构，显示出一个厚的、快速增生的年轻礁体（最大年龄900 cal yr BP）。珊瑚礁上的活珊瑚覆盖面积随着离礁顶的距离而增加，但在20米的深度处终止，在那里珊瑚礁结束于一个大的沙场。垂直珊瑚礁生长的主要限制是波基下的容纳空间，而不是招募活动或能量条件。对岩心岩相的解释表明，莫洛凯岛西南角的现代珊瑚礁生长，以及整个夏威夷的延伸，是由与北太平洋折射膨胀有关的波浪引起的近床剪切应力控制的。全新世吸积模式也反映了晚全新世北涌波引起的近层剪应力的长期影响。这一发现与其他研究（例如，Grigg 1998; Cabioch et al. 1999）一致，这些研究反映了在夏威夷其他地方以及整个太平洋和印度洋的现代和晚全新世增生中，涌浪能量和海平面占主导地位。然而，值得注意的是，这一结果在Rooney等人（2003）的假设中得到了完善和澄清，该假设指出，大约5000年前开始的厄尔尼诺南方涛动的增强导致了北涌浪能量的增加，并标志着夏威夷裸露海岸线净增长的结束。Hale O Lono暴露于北涌浪和那里的海底年龄（约4800 calyr BP），再加上Hikauhi没有北涌浪的发生，以及过去一千年来在那里发生的持续增生，有力地支持了Rooney等人（2003）提出的ENSO珊瑚礁假说。控制研究点全新世礁体增生的因素还包括相对海平面位置和上升速度，以及劳点的挡浪作用。莫洛凯岛西南海岸适合珊瑚礁增生的栖息地在整个全新世都在萎缩。
ABSTRACT Two reef systems off south Molokai, Hale O Lono and Hikauhi (separated by only 10 km), show strong and fundamental differences in modern ecosystem structure and Holocene accretion history that reflect the influence of wave-induced near-bed shear stresses on reef development in Hawaii. Both sites are exposed to similar impacts from south, Kona, and trade-wind swell. However, the Hale O Lono site is exposed to north swell and the Hikuahi site is not. As a result, the reef at Hale O Lono records no late Holocene net accretion while the reef at Hikauhi records consistent and robust accretion over late Holocene time. Analysis and dating of 24 cores from Hale O Lono and Hikauhi reveal the presence of five major lithofacies that reflect paleo-environmental conditions. In order of decreasing depositional energy they are: (1) coral-algal bindstone; (2) mixed skeletal rudstone; (3) massive coral framestone; (4) unconsolidated floatstone; and (5) branching coral framestone-bafflestone. At Hale O Lono, 10 cores document a backstepping reef ranging from 8,100 cal yr BP (offshore) to 4,800 cal yr BP (nearshore). A depauperate community of modern coral diminishes shoreward and seaward of 15 m depth due to wave energy, disrupted recruitment activities, and physical abrasion. Evidence suggests a change from conditions conducive to accretion during the early Holocene to conditions detrimental to accretion in the late Holocene. Reef structure at Hikauhi, reconstructed from 14 cores, reveals a thick, rapidly accreting and young reef (maximum age 900 cal yr BP). Living coral cover on this reef increases seaward with distance from the reef crest but terminates at a depth of 20 m where the reef ends in a large sand field. The primary limitation on vertical reef growth is accommodation space under wave base, not recruitment activities or energy conditions. Interpretations of cored lithofacies suggest that modern reef growth on the southwest corner of Molokai, and by extension across Hawaii in general, is controlled by wave-induced near-bed shear stress related to refracted North Pacific swell. Holocene accretion patterns here also reflect the long-term influence of wave-induced near-bed shear stress from north swell during late Holocene time. This finding is consistent with other studies (e.g., Grigg 1998; Cabioch et al. 1999) that reflect the dominance of swell energy and sea level in controlling modern and late Holocene accretion elsewhere in Hawaii and across the Pacific and Indian oceans. Notably, however, this result is refined and clarified for Hawaii in the hypothesis of Rooney et al. (2003) stating that enhancement of the El Nino Southern Oscillation beginning approximately 5000 years ago led to increased north swell energy and signaled the end to net accretion along exposed coastlines in Hawaii. The exposure of Hale O Lono to north swell and the age of sea floor there (ca. 4,800 cal yr BP), coupled with the lack of north swell incidence at Hikauhi and the continuous accretion that has occurred there over the last millennium, strongly supports the ENSO reef hypothesis as outlined by Rooney et al. (2003). Other factors controlling Holocene reef accretion at the study site are relative sea-level position and rate of rise, and wave sheltering by Laau Point. Habitat suitable for reef accretion on the southwest shore of Molokai has shrunk throughout the Holocene.