Geologic-hydrologic setting and surface sedimentology in the Persian Gulf

1. Morphology and sedimentation
The deepest parts of the Persian Gulf lie off the Iranian coast. Several swells separate the Persian Gulf into the Western Basin, the Central Basin and the Strait of Hormuz, which leads without noticeable morphological interruption onto the Biaban Shelf; the latter gradually drops off towards the continental slope, which itself has a strongly subdivided morphology.
The sediment distribution in the Western Basin runs parallel to the basin's axis to a depth of 50 -60 m. This is caused by the shallow and uniform slope of the Iranian coast into the Western Basin, by clear exposure of the area to the Shamal-Winds and by tidal currents parallel to the basin's axis. Most other parameters also show isolines parallel to the coast line. Data from the sediment analyses show a net transport which extends out along the Central Swell: coarse fraction > 63 µ, total carbonate content, carbonate in fine fractions < 2 µ, 2-6 µ and 20-63 µ, calcite-aragonite ratios in the fine fractions 2-6 µ and 20-63 µ and quartz-dolomite ratios in fine fraction 2-6 µ. At least the uppermost 10-40 m of this sediment is late Holocene. This implies sedimentation rates of several meters per 1000 years.
The slope from the Iranian coast into the Central Basin (max. depth 100 m) is generally steeper, with interspersed islands and flats. Both facts tend to disturb a sediment dustribition parallel to the basin's axis over extensive areas and may preclude any such trend from being detected by the methods and sample net used.
The spatial distribution of the coarse fraction, however, seems to indicate sediment transport at greater water depths perpendicular to the basin's long axis and along the steepest gradients well into the Central Basin.
The flats of the Central Basin have a sediment cover distinctly different from those of the deeper basin areas. Characteristic parameters are the extremely high percentages of coarse grained sediments, total content of carbonate CO2 over 40, low total organic carbon content, (however values are high if calculated on the basis of the < 63 µ fraction), low total N-content, and low C/N ratios. These characteristics probably result from the absence of any terrigenous material being brought in as well as from exposure to wave action. Finest terrigenous material is deposited in the innermost protected part of the Hormuz Bay.
In the deep channel cut into the Biaban Shelf which carries the Persian Gulf out-flow water to the Indian Ocean, no fine grained sediment is deposited as shown by grain size data.
2. Geographic settings and sedimentation
Flat lands border the Arabian coast of the Persian Gulf except for the Oman region. The high and steep Zagros Mountains form the Iranian coastline. Flat topography in combination with generally low precipitation precludes fluviatile sediment being added from the South. Inorganic and biogenic carbonates accumulating under low sedimentation rates are dominant on the shallow Arabic Shelf and the slopes into the Western and Central Basins. The fluviatile sediment brought in from the Iranian side, however decisively determine the composition of the Holocene sediment cover in the Persian Gulf and on the Biaban Shelf. Holocene sediments extend 20-30 km seaward into the Western Basin and about 25 km on to the Biaban Shelf. As mentioned before, sedimentation rates are of several meters/1000 years.
The rocks exposed in the hinterland influence the sediments. According to our data the Redbeds of the Zagros Mountains determine the colour of the very fine grained sediments near the Iranian Coast of the Persian Gulf. To the West of Hormuz, addition of carbonate minerals is particularly high. Dolomite and protodolomite, deposited only in this area, as well as palygorskite, have proven to be excellent trace minerals.
To the East of Hormuz, the supply of terrigenous carbonates is considerably lower. Clay minerals appear to bring in inorganically bound nitrogen thus lowering the C/N ratio in these sediments especially off river mouths.
3. Climate and sedimentation
The Persian Gulf is located in a climatically arid region. This directly affects sedimentation through increased wind action and the infrequent but heavy rainfalls which cause flash floods. Such flash floods could be responsible for transporting sedheats into the Central Basin in a direction perpendicular to the Gulf's axis.
Eolian influx is difficult to asses from our data; however, it probably is of minor importance from the Iranian side and may add, at the most, a few centimeters of fine sediment per 1000 years.
4. Hydrology and sedimentation
High water temperatures favor inorganic carbonate precipitation in southern margin of the Gulf, and probably on the flats, as well as biogenic carbonate production in general.
High evaporation plus low water inflow through rivers and precipitation cause a circulation pattern that is typical for epicontinental seas within the arid climate region. Surface water flows in from the adjoining ocean, in this case the Indian Ocean and sinks to the bottom of the Persian Gulf mainly in the northern part of the Western Basin, on the "Mesopotamischer Flachschelf" ard probably in the area of the "Arabischer Flachschelf". This sinking water continually rejuvenates the bottom out-flow water. The inflowing surface water from the Indian Ocean brings organic matter into the Persian Gulf, additional nutrients are added by the "fresh" upwelling waters of the Gulf of Oman. Both nutrients and organic matter diminish very rapidly as the water moves into the Persian Gulf. This depletion of nutrients and organic matter is the reasonfor generally low organic carbon contents of the Persian Gulf sediments. The Central Swell represents a distinct boundary, to the west of which the organic carbon content are lower than to the east when sediment samples of similar grain size distribution are compared.
The outflow carries well oxygenated water over the bottom of the Persian Gulf and the resulting oxidation further decreases the content of organic matter. In the Masandam-Channel and in the Biaban-Shelf channel, the outflowing water prevents deposition of fine material and transports sediment particles well beyond the shelf margin.
The outflowing water remains at a depth of 200-300 m depending on its density and releases ist suspending sediment load to the ocean floor, irrespectative of the bottom morphology. This is reflected in several parameters in which the sediments from beneath the outflow differ from nearby sediments not affected by the outflowing water. High carbonate content of total samples and of the individual size fraction as well as high aragonite and dolomite contents of individual size fractions characterize the sediment beneath the outflowing water.
The tidal currents, which avt more or less parallel to the Gulf's axis, favor mixing of the water masses, they rework sediments at velocities reported here. This fact enlarges to a certain degree the extent of our interfaces which are based on only a few sample points (Persian Gulf and Biaban Shelf one sample per 620 km**2, continental slope one sample per 1000 km**2). The water on the continental slope shows and oxygen minimum at 200-1200 m which favors preservation of organically-bound carbon in the sediment. The low pH-values may even permit dissolution of carbonate minerals.

1. 地貌与沉积作用
波斯湾最深水域位于伊朗沿岸海域。数条隆起将波斯湾划分为西部盆地、中部盆地与霍尔木兹海峡（Strait of Hormuz），后者无明显地貌间断直接衔接比班陆架（Biaban Shelf）；比班陆架向大陆坡方向逐渐倾斜，而大陆坡本身具有高度分异的地貌形态。

西部盆地的沉积物分布与盆地轴线平行，延伸至50~60米水深。这一分布格局由伊朗沿岸向西部盆地的平缓均一斜坡、该区域直接受沙马尔风（Shamal-Winds）侵蚀、以及平行于盆地轴线的潮流共同塑造。其余多数参数的等值线也与海岸线平行。沉积物分析数据显示，沿中央隆起向外延伸存在净搬运作用，涉及的参数包括：粒径>63μm的粗粒组分、总碳酸盐含量、粒径<2μm、2~6μm及20~63μm细粒级中的碳酸盐含量、粒径2~6μm和20~63μm细粒级中的方解石-文石比值，以及粒径2~6μm细粒级中的石英-白云石比值。该套沉积物至少上部10~40米属于晚全新世（late Holocene），据此推算沉积速率可达数米/千年。

伊朗沿岸向中部盆地（最大水深100米）的斜坡通常更陡峭，且散布有岛屿与浅滩。这两个因素会在大范围区域内破坏平行于盆地轴线的沉积物分布格局，可能导致现有采样方法与网格无法识别此类分布趋势。

但粗粒组分的空间分布似乎表明，在更大水深区域，沉积物搬运方向垂直于盆地长轴，并沿最陡梯度向中部盆地内部延伸。

中部盆地的浅滩沉积物覆盖与更深盆地区域显著不同，其特征参数包括：粗粒沉积物占比极高、总碳酸盐CO₂含量超过40%、总有机碳含量较低（但若以<63μm粒级为基准计算则数值偏高）、总氮含量较低，以及C/N比值偏低。这些特征可能源于无陆源物质输入，同时受波浪作用影响。最细的陆源物质仅沉积在霍尔木兹湾最内侧的遮蔽区域。

在切入比班陆架、承载波斯湾流出水进入印度洋的深水水道中，粒度数据显示无细粒沉积物沉积。

2. 地理背景与沉积作用
波斯湾阿拉伯沿岸除阿曼地区外，均为平坦陆地环绕。高耸陡峭的扎格罗斯山脉（Zagros Mountains）构成了伊朗海岸线。平坦的地形配合普遍偏低的降水量，使得南部无河流沉积物输入。在浅海阿拉伯陆架及向西部、中部盆地延伸的斜坡上，以低沉积速率堆积的无机与生物碳酸盐为主。但伊朗侧输入的河流沉积物，却决定性地决定了波斯湾及比班陆架全新世沉积物覆盖层的组成。全新世沉积物在西部盆地向海延伸20~30公里，在比班陆架上延伸约25公里。如前所述，沉积速率可达数米/千年。

内陆出露的岩石会影响沉积物特征。根据我们的数据，扎格罗斯山脉的红层决定了波斯湾伊朗沿岸附近极细粒沉积物的颜色。在霍尔木兹以西区域，碳酸盐矿物的输入量尤其高。仅在该区域沉积的白云石、原白云石以及坡缕石（palygorskite）已被证实为优质示踪矿物。

霍尔木兹以东区域，陆源碳酸盐的输入量显著降低。黏土矿物似乎携带着无机结合态氮，从而降低了这些沉积物的C/N比值，在河口附近尤为明显。

3. 气候与沉积作用
波斯湾地处干旱气候区。干旱通过增强风力作用，以及罕见但强度较大的降雨引发的山洪，直接影响沉积过程。此类山洪可能将沉积物以垂直于海湾轴线的方向搬运至中部盆地。

风成输入量难以通过现有数据评估，但从伊朗侧来看其重要性较低，最多每千年仅能添加数厘米的细粒沉积物。

4. 水文与沉积作用
较高的水温有利于海湾南部边缘及浅滩区域的无机碳酸盐沉淀，总体上也促进了生物碳酸盐的生成。

高蒸发速率加上河流输入与降水量的匮乏，形成了干旱气候区陆表海典型的环流模式。表层水从相邻海域——即印度洋——流入，并主要在西部盆地北部、美索不达米亚浅陆架（Mesopotamischer Flachschelf）及可能的阿拉伯浅陆架（Arabischer Flachschelf）区域下沉。下沉的水体持续更新底层流出水。从印度洋流入的表层水将有机质带入波斯湾，阿曼湾的“新鲜”上升流也会补充额外营养盐。随着水体进入波斯湾，营养盐与有机质含量迅速降低。这种营养盐与有机质的消耗，是波斯湾沉积物有机碳含量普遍偏低的原因。中央隆起构成了明显的边界：当比较粒度分布相似的沉积物样品时，其西侧的有机碳含量低于东侧。

流出水携带着富氧水体流经波斯湾底层，由此产生的氧化作用进一步降低了有机质含量。在马桑达姆水道（Masandam-Channel）与比班陆架水道中，流出水阻碍了细粒物质的沉积，并将沉积物颗粒搬运至陆架边缘以外更远区域。

流出水的密度决定其停留深度为200~300米，并将悬浮沉积物载荷沉积至海底，不受海底地貌影响。这一点在多项参数中有所体现：流出水体下方的沉积物与未受其影响的邻近沉积物存在显著差异。流出水体下方的沉积物特征包括：总样品及各粒级的碳酸盐含量较高，以及各粒级中的文石与白云石含量较高。

潮流大致平行于海湾轴线流动，有利于水团混合，其作用强度据本文报道的流速可对沉积物进行再改造。这一因素在一定程度上扩大了我们基于少量采样点绘制的界面范围（波斯湾与比班陆架的采样密度为每620平方公里1个样品，大陆坡为每1000平方公里1个样品）。大陆坡水域在200~1200米深度存在氧最小值层，有利于沉积物中有机结合碳的保存。较低的pH值甚至可能促进碳酸盐矿物的溶解。