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Large Groundwater Strontium Flux to the Oceans from the Bengal Basin and the Marine Strontium Isotope Record

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Science  24 Aug 2001:
Vol. 293, Issue 5534, pp. 1470-1473
DOI: 10.1126/science.1060524

Abstract

Strontium concentration and isotopic data for subsurface flowing groundwaters of the Ganges-Brahmaputra (G-B) delta in the Bengal Basin demonstrate that this is a potentially significant source of strontium to the oceans, equal in magnitude to the dissolved strontium concentration carried to the oceans by the G-B river waters. The strontium concentrations of groundwaters are higher by a factor of about 10 than typical G-B river waters and they have similar87Sr/86Sr ratio to the river waters. These new data suggest that the present contribution of the G-B system to the rise in 87Sr/86Sr ratio in seawater is higher by at least a factor of 2 to 5 than the average over the past 40 million years.

Variations of87Sr/86Sr ratio in seawater are primarily due to variations in continental erosion rates as well as variations in the Sr isotopic composition of this input to the oceans (1, 2). Thus, changes in this isotopic ratio are frequently linked to major tectonic events (3, 4) and have been used to constrain the problem of Snowball Earth glaciations (5). The87Sr/86Sr ratio of seawater has increased from about 0.7078 to 0.7092 over the past 40 million years (My). This rapid increase has been attributed to the Himalayan uplift, following the India-Asia continental collision (2), which is believed to have contributed Sr with particularly high87Sr/86Sr ratios to the oceans through the major rivers draining the Himalayas (3, 4, 6, 7). The Himalayan uplift has also been suggested as the major cause of the global climate cooling for the past 40 My, because of the increase in chemical weathering of silicates and the resulting decrease of atmospheric CO2 concentration (8).

Most workers accept the importance of the contribution of Himalayan river water dissolved Sr for the marine budget, although the sources and the amount of this Sr flux remain problematic (9). Various calculations based on Sr isotopic compositions and Sr concentrations of the G-B rivers indicate that the Himalayan discharge accounts for a large fraction of the observed increase in the marine87Sr/86Sr ratio over the past 40 My (4,6). However, a study of Himalayan riverine187Os/188Os (10) could not explain the observed correlated Sr-Os isotopic shift in the marine record by Himalayan input. There is also uncertainty whether the Sr flux and its isotopic composition are primarily controlled by the weathering of silicate minerals (6, 9, 11, 12) or are controlled by weathering of metacarbonates that reequilibrated with silicates with high 87Sr/86Sr during metamorphism (7,13, 14), or both.

Most earlier studies focused on the weathering of high Himalayan rocks. Here, we are concerned with the effect of groundwater-sediment interaction processes in the G-B flood plain of the Bengal Basin (Fig. 1) in controlling the budget of oceanic Sr as well as the Sr isotopic composition of seawater. This study was prompted by the recent discovery of large groundwater flux on a regional scale to coastal waters (15) and that desorption of226Ra and Ba from deltaic sediments in this area is a significant source of these elements in the waters of the Bay of Bengal (16). There is clear evidence of a large groundwater discharge with high Ra and Ba fluxes to the ocean from the G-B rivers during low river discharge (17).

Figure 1

Map of the Bengal Basin showing the main courses of the Ganges-Brahmaputra and their tributaries and the sample locations for this study. River water locations are shown by crosses, and the groundwater sites are shown by filled symbols. Townships are in open squares.

For this study, we collected 61 groundwater samples from the southern part of Bangladesh between the Ganges (Padma), Brahmaputra, and Meghna rivers and their tributaries, and nine groundwater samples just north and south of Calcutta in the western part of the basin (Fig. 1). The water samples come from depths of 10 to 350 m. We also collected river waters from six sites of the G-B system within and adjacent to the Bengal Basin (Fig. 1 and Table 1). All the river and groundwater samples were collected during the dry season in January through May and were filtered (0.2 μm pore size) and acidified on site. The Sr concentration and87Sr/86Sr ratios are reported in Table 1. For individual sites where more than one groundwater sample was analyzed from different depths, Table 1 gives the range and average values for Sr. For many of the groundwater samples, we also examined other chemical characteristics, such as dissolved cations and anions, carbon, oxygen, and hydrogen isotopes for all samples, and3H-3He ages for several of the samples. Here, we are concerned mostly with the 87Sr/86Sr and Sr concentrations in these waters, and implications of a groundwater Sr flux on the marine Sr budget.

Table 1

Sr-isotopic compositions and concentrations (C Sr) in Bengal Basin groundwaters and river waters. Where more than one sample was analyzed, averages and ranges are shown. n is number of samples measured. Strontium concentrations were measured by inductively coupled plasma mass spectrometry and the 87Sr/86Sr by thermal ionization mass spectrometry, both at the University of Rochester. Errors in Sr concentration determinations are usually less than 3% and for individual Sr isotope ratio measurements, normalized to86Sr/88Sr = 0.1194, the errors in 2σ of the mean correspond to less than 3 in the fifth decimal place. The NBS 987 Sr standard gave 87Sr/86Sr = 0.710246 ± 28 (n = 8) during the course of these measurements.

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The Sr concentrations in the groundwater show a variable range, with a significant number of samples falling in the range of 4 to 6 μmol/liter. Some samples, which are not included in Table 1, show much higher concentrations (14 to 53 μmol/liter) and are from localities near the coastal regions. As shown by their high Cl content, these very high Sr concentrations are due to mixing with seawater. However, all the other groundwaters listed in Table 1 have been shown to be free of a seawater component, because of their low and uniform Cl contents. In contrast with the groundwaters, the Bengal Basin river waters show much lower concentrations of Sr (∼0.5 μmol/liter), similar to or higher than typical Sr contents of the G-B rivers and their tributaries outside the Bengal Basin (9, 18). From the average Sr flux of Galy et al. (9) and the average water flux used by Galy and France-Lanord (19), we obtain an average Sr concentration for the G-B river system of 0.61 μmol/liter. Because these rivers discharge into the oceans through the Bengal Basin delta, the flood plain waters should be of utmost significance in estimating the isotopic composition of the Sr flux to the ocean. The strong seasonal variability and low Sr contents of the river water during the monsoon (9) should also be considered. Overall, we estimate from our measurements of the Bengal flood plains (Table 1) that the groundwater Sr concentrations are about 10 times higher than that of the river waters.

Next, we compared the 87Sr/86Sr compositions of the Bengal Basin groundwater with those of the adjacent surface water flow in the rivers (Fig. 2). The87Sr/86Sr ratios in waters of the Bengal Basin rivers are within the same range as those of the groundwater (Table 1 and Fig. 2). Seasonal variability in the G-B rivers has recently been noted (9); in particular these rivers appear to show low Sr concentrations and higher87Sr/86Sr ratios during the monsoon (9) as shown in Fig. 2. In general, higher87Sr/86Sr ratios and lower concentrations of Sr are the characteristic features of the Himalayan drainage outside the Bengal Basin (3, 6, 7, 9, 1114). The Bengal Basin flood plain, where the surface discharge has similar87Sr/86Sr signatures as those of the groundwaters, deserves further scrutiny because the rivers may show generally higher or similar Sr concentrations compared to those outside the basin, as reported by previous workers and summarized in Galyet al. (9). Note also that in Table 1, coastal groundwaters and river waters, as well as the Brahmaputra, have similar87Sr/86Sr (typically ∼0.717 to 0.720), and inland groundwaters are similar to the Ganges and its tributary (typically ∼0.720 to 0.730) as measured in two sites near Farakka and Calcutta inside the basin (Fig. 1). The very high87Sr/86Sr ratios in Himalayan headwaters of the Ganges are not seen in the Bengal Basin. Most of the variability in groundwater 87Sr/86Sr ratio is seen at shallow depths of up to 60 m; below this depth, there is a tendency for the water to have a narrow range in 87Sr/86Sr composition between 0.715 to 0.725.

Figure 2

Strontium concentrations and87Sr/86Sr of groundwaters and river waters of the Bengal Basin. These waters are used to characterize the present-day discharge of Sr to the oceans via the G-B flood plain. Groundwaters and river waters are averages from Table 1, except for the monsoon values (open squares) of Galy et al. (9). The fields showing the range of measured values include both our new data as well as the data summarized by Galy et al. (9), except for the river water measurements in the monsoon season.

For the present study, an estimate of groundwater annual recharge rate can be made from our 3H-3He isotopic ages (20, 21). An average recharge estimate of 0.6 ± 0.2 m/year (21) over the entire G-B flood plain (300,000 km2) translates into a steady-state groundwater flux to the Bay of Bengal of 0.2 × 1015 liter/year, equal to ∼19% of the surface water flux (1.07 × 1015 liter/year) to the Bay of Bengal. Independent confirmation of the magnitude of this groundwater discharge comes from the agreement between the Ba flux obtained by multiplying our observed groundwater Ba average of 1.2 μmol/liter with the groundwater (G-B) flux (= 2.4 × 108 mol/year) and the Ba discharge given by Moore (17) based on the correlated Ba-Ra excesses in the Bay of Bengal (3 × 108 to 30 × 108 mol/year). The fact that our groundwater estimate falls at the low end of the estimate by Moore indicates that we are not severely overestimating the magnitude of this discharge.

The first part of Table 2 gives the Sr isotope balance for the G-B rivers. From our new data in Table 1, in addition to the Galy et al. data, it is clear that the average Sr isotopic composition of the G-B rivers must be between the high value of 0.7295 favored by Galy et al. (9) and our much lower value for the Brahmaputra (0.717). The higher87Sr/86Sr value of Galy et al.(9) reflects, in part, their use of data from the monsoon season when the values can be highly variable. Because of the highly variable 87Sr/86Sr in the G-B river system, it is difficult to arrive at a reliable average87Sr/86Sr value; in the following, we carry out calculations both with our new lower value (0.717) as well as that of Galy et al. (9) (∼0.73). We use the total G-B river water Sr flux of 0.65 × 109 mol/year from Galyet al. (9).

Table 2

The Ganges-Brahmaputra river and groundwater Sr isotopic balance and the global marine Sr isotopic mass balance. The total mass of Sr in seawater is 1.25 × 1017mol.

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Using our new groundwater data, we obtain an average dissolved Sr content of 4.5 μmol/liter. We thus obtain an additional subsurface groundwater Sr flux of 0.9 × 1015 μmol/year, which is about 1.4 times the combined G-B riverine Sr flux to the oceans (Table 2). As shown in Table 1, many of the coastal groundwaters have a 87Sr/86Sr of about 0.715, whereas inland values are typically somewhat higher than 0.72. Thus, a likely range of87Sr/86Sr ratio in groundwater to the ocean is 0.715 to 0.720. Combining these, we obtain a total flux (river and groundwater) from this system to the oceans for Sr of 1.55 × 109 mol/year, with 87Sr/86Sr in the range of ∼0.716 to 0.724. These results document the important contributions of groundwater discharge of Sr through the Bengal flood plain as a significant portion of the Himalayan drainage, and this flux must be taken into account in any model for the global exogenic Sr cycle.

In the second part of Table 2, we use published (1, 4, 7, 9, 19,22) as well as our new estimates for the global Sr cycle. The total G-B flux compared to the global continental flux of Sr to the oceans is J GB/J total= 0.031, or ∼3%. This increased flux results in a new and shorter oceanic residence time for Sr of τSr = 2 My. Thus, use of the groundwater estimates in Table 2 suggests that the global continental Sr flux is ∼50% higher than that used before.

The global cycle is modeled using the following87Sr/86Sr (= αSr) isotope mass balanceEmbedded Image(1)where J i is the Sr flux from reservoir i, αSr-i is the Sr isotopic composition of this flux, SW is seawater, andN Sr is the mass of Sr in the oceans. Estimates of the present values of theJ iSr-i – αSr-SW) terms are given in the last column of Table 2. Note that although the G-B flux is only 3% of the total continental Sr flux to the oceans, it accounts for between 11% of the global isotope balance using the low87Sr/86Sr value (0.7158) and 26% using the high value of 87Sr/86Sr (0.7242). Over the past 40 My, seafloor spreading rates were essentially constant (23). Thus, the hydrothermal contribution to the Sr cycle was most likely constant over this period of time, and the reason for the global rise in 87Sr/86Sr must be either an increase in the total continental flux of Sr or a change in its isotopic composition, or both. The contribution of G-B to the global cycleEmbedded Image(2)is equal to 0.82 × 10−4My−1 for the low estimate of87Sr/86Sr and 1.86 × 10−4My−1 for the high estimate of87Sr/86Sr in Table 2. This rate of change is a factor of ∼2.3 to 5.3 higher than the observed average value of dαSr-SW/dt ∼0.35 × 10−4My−1 for the past 40 My.

We also note that use of a 87Sr/86Sr value of 0.711 for global river and continental flux creates an imbalance in the Sr cycle. To rectify this situation, we need to lower the continental flux isotopic composition to about 0.71049 [similar to the value proposed in (1)]. Also, the additional global continental Sr flux from groundwater would cause a rise in87Sr/86Sr of 0.0095 over 40 My if left unbalanced. This is higher by a factor of 7 than the observed rise over the past 40 My.

Thus, we conclude that the groundwater data have an enormous effect on the interpretation of the seawater Sr isotope balance. Although we do not claim that the new values presented in Table 2should be considered as final, these data urge caution about overinterpreting Sr isotope data from a few local watersheds in this area. For example, trying to use the seawater Sr isotope curve to infer the detailed tectonic uplift history of the Himalayas as well as for estimating effects on global climate change still involves considerable uncertainty. Because of the highly variable nature of87Sr/86Sr in the G-B river system, reliable average values are difficult to estimate.

  • * To whom correspondence should be addressed. E-mail: abasu{at}earth.rochester.edu

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